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Gemstone Toughness


#1

In my current gemological studies, I learned that gemstones possess
two separate durability components, hardness and toughness. Hardness
is the stones resistance to scratching, while toughness is its
resistance to chipping, breaking or cracking. Diamond is a good
example. Diamond is the hardness known natural stone and yet it is
rated only “good” for toughness. As I have studied, Jade is tougher
than diamond.

There has been a discussion recently on the wearability of gemstones.
Diamond, Corundum and Tourmaline are the stones mentioned. For
reference, here is a list of some gemstones with their respective
hardness and toughness ratings.

   Hardness     Toughness

Diamond 10 Good
Corundum 9 Excellent
Topaz 8 Poor
Tourmaline 7.5 Fair
Jade 7 Excellent
Opal 6 Very Poor

Now, I realize that numbers from a lab and performance in everyday
life can be two different things. For that very reason I am writing
this inquiry. When faceted edges are abraded, it is the wearing away
and chipping of these edges that produces this effect. It seems to
me that toughness plays a greater part in this effect than hardness.
From the toughness ratings for the stones listed, it appears that
Corundum and Jade would resist the everyday abuses of wear better
than the others, even better than Diamond. However, from previous
discussions, the very opposite may be true. Which is correct from a
practical observation?

I am calling upon all the experience out there to answer this
question. Experience is the most important ingredient of the
professional; an ingredient I lack. I consider myself fortunate to
be able to have a group like Orchid to learn from. Thank you.

Charles Heick


#2

Charles - I don’t do diamond work, so cannot testify as to the
toughness issues there. However, both corundum and jade are
monstrously tough. The practical test for rubies in days of auld was
attaching them to the horns of mountain sheep, and letting them butt
away. As far as jade is concerned - some of the original hammerstones
and anvils for working metal were nephrite jade. Once you’re past the
initial “toughness” issue you must also deal with facet edges vs.
cabochon surfaces. One of the most frequent complaints I receive from
women is how dull their ring stones appear because the edges of the
facets have been abraded to a curved, sand-blasted finish. A major
culprit is close-fitting leather gloves. Even on “tough” stones,
facet meets are subject to extreme abrasion, which should be kept in
mind when mountings are designed and created.

Jim Small
Small Wonders


#3
From the toughness ratings for the stones listed, it appears that
Corundum and Jade would resist the everyday abuses of wear better
than the others, even better than Diamond.  However, from previous
discussions, the very opposite may be true.  Which is correct from a
practical observation?

G’day; it my personal experience that although I agree that jade is
tough (more later) It does not wear too well. I used to make
sterling rings set with highly polished jade cab stones. Six month’s
later those rings looked pretty terrible - they were badly dulled,
and it was a job to repolish them for it was scarcely worth the
bother of unsetting them from a bezel. So I stopped making them.

However, as a measure of the toughness of jade, I have watched a
Maori doing complicated and large scale carvings in Totara (a NZ
native timber) and the shavings were coming off his jade chisels and
gouges (Maoris call it pounamu; green stone) exactly as though they
were first class steel - and they seemed to stay sharp. Now try that
with almost any other stone! – John Burgess; @John_Burgess2 of
Mapua Nelson NZ


#4

Dear Charles; It seems that there is always a lot of disagreement
between academics and people who have experience with working
substances. Theoretically Topaz should be brittle, but in reality it
is quite durable. Corundum , while being quite tough and heat
resistant, can be quite susceptible to heat shock. ( Yes, I read your
addendum) Furthermore, when it comes to bruising of the facet
convergences, ruby and sapphire can be very susceptible to damage. As
for diamond, I hardly think that it warrants classification as a mere
"good" with respect to toughness. When you consider the fact that
diamond crystals are often soldered onto a steel rod for use in
truing grinding wheels you could hardly consider that to be a timid
use! No, I am not referring to its ability to abrade all other
substances…I am referring to its ability to withstand the
continuous impact of the crystal against a rapidly rotating rigid
substance ! In my experience I would alter the toughness ratings of
your first three stones as follows: Change Diamond from good to
outstanding; change Corundum from excellent to good and change Topaz
from poor to good. I commend you on your good logic and willingness to
reexamine that which is taken for granted. As I have said before, it
is lamentable that many assumptions often turn out to be the
unquestioning compounding of previous assumptions: when I see a long
list of credits in an academic treatise I wonder how much
consideration has been given to the possibility that the author’s
predecessors were full of malarkey. It harks back to the old yin/yang
of author vs. authority. You may write very well inspite of not
having any real knowledge of your subject. Ron at Mills Gem, Los
Osos, CA.


#5

Hello Charles,

A small addition to your article.The hardness of diamonds are not
exactly 10.It has somekind of “window”.Diamond can be 9.5 as others
stones can be a little harder of softer as the numbers you
mentioned.If diamonds had a hardness of 10 then they wouldn’t be
cutable since stones of the same hardness do not scratch eachother.

Regards Pedro
Palonso@t-online.de


#6

Dear Pedro, I hope you are going to be ready for all the comments you
are going to get about your latest dissertation on Diamond hardness.
I am afraid you have made a number of statements that are quite
unfounded. Your statement that diamond can be 9 1/2 on the Moh’s
scale is dead wrong. The actual hardness of diamond on an adjusted
Moh’s scale is between 37 and 43 ! Furthermore, it is quite possible
for minerals of the same hardness to scratch one another. Indeed, how
do you suppose diamonds are cut? Aside from vaporization by laser
beam, diamond is the ONLY substance that can abrade a diamond.! I
have always admired your youthful enthusiasm Pedro, but I am afraid
that this time you have flunked the exam ! Better luck next
time…Ron at Mills Gem, Los Osos, CA.


#7

Hello Charles, It is true that hardness and toughness are two
different animals. Yet both are a “measure” of the stone’s resistance
to change and are placed under the heading of cohesion. Many factors
come into play including the methods used to measure these phenomena.
Lets start with hardness, the resistance to abrasion. The method used
by Mohs is a relative scale, a comparison of the ability of one stone
to scratch another. The scale was set 1 to 10, 10 being the hardest.
Using this method corundum is placed at 9. However, using methods to
measure absolute hardness, diamond can be as much as 40 times harder
than corundum. Furthermore, different directions and planes within
most crystals will have different hardness. The hardest surface known
is the {111} cleavage surface of a diamond. Diamond chips and dust
will scratch and polish all surfaces except the {111} surface of other
diamonds. When discussing hardness we are really talking about
breaking bonds between atoms and molecules. This can be a very complex
subject, but the Mohs scale just keeps things simple for us
non-material scientists. Toughness also has many variables, hardness,
crystal structure, atomic makeup, impurities, are a few. In the case
of the jades, toughness is due to the interlocking of
fibers,…imagine the interlocking fibers of cellulose in paper.
Cellulose is not very hard, but try breaking a phone book with a
hammer.

There are other factors that contribute to the wearing away of
faceted edges and points (sometimes referred to as “paper worn”). Some
crystals are formed under conditions of great heat and pressure. When
these are brought to the earth’s surface, the new set of
pressure/temperatures may cause adjustments to the stone’s atomic
structure and produce weakness, particularly on edges and points.

Here’s an interesting one, there is a large boulder of nephrite jade
on display at the California Academy of Sciences in San Francisco.
Over the years children (and some adults) will mount it and slide down
it’s smooth surface. This activity has produced a fine blue jean
polish on the boulder. How can this be, blue jeans are much softer
than jade?

Regards, Will Estavillo


#8

Hello Pedro. Thank you, and all the others who responded to my
inquiry concerning gemstone toughness. Yes Pedro, diamonds do vary
in hardness depending on their molecular purity. An industrial
quality diamond containing quantities of other molecules within its
crystal structure would, of varying degrees, be relatively softer
than a diamond of gem quality. In terms of toughness, an imperfect
diamond containing many inclusions may be as hard as a flawless one,
but would lack its toughness. For this reason, alluvial diamonds are
generally of a better quality overall than those mined.

However, with all due respect, I disagree when you said “since stones
of the same hardness do not scratch each other.” Materials of the
same hardness do scratch each other and this is an accepted fact in
mineralogy and surely in other disciplines. In reference to
diamonds, the stones in a paper of melee can become scratched and
abraded when handled too often. Larger cut diamonds should always be
separated into individual papers to prevent them from scratching and
abrading each other. Consider this, if diamonds of the same hardness
could not scratch each other, how would a D-flawless diamond be cut
and polished?

Please Pedro, take this in the spirit it’s being presented. I wish
not to offend you or the assistance you so graciously volunteered.

Charles Heick


#9

“This activity has produced a fine blue jean polish on the boulder.
How can this be, blue jeans are much softer than jade?”

Hello Will. Thanks for your reply on gemstone hardness. In answer
to your observation, I would say that the polish on the Jade from
sliding over it was produced not from the jeans themselves, but from
the dirt and grit that was on and locked into the jean fabric. Most
earthly grit is composed of silicates which are harder than Nephrite
Jade. This is the same effect as grit on a polishing lap; with the
jean fabric acting as the lap.

“Diamond chips and dust will scratch and polish all surfaces except
the {111} surface of other diamonds.”

Concerning the crystallographic planes of a diamond, the cleavage
plane or octahedral plane is, as you wrote, the hardest polishing
direction. However, it can be polished with much difficulty. For
that reason, the softest direction, the dodecahedral face, and the
cubic face with intermediate hardness are preferred.

“However, using methods to measure absolute hardness, diamond can be
as much as 40 times harder than corundum.”

On the linear Sclerometer scale, diamond has a hardness of 140,000
with corundum rated at 1,000. This would make the diamond 140 times
harder on this reference.

I have to admit, Orchid is being a great asset to my education.
Writing about a subject is much more academically reinforcing than
just reading a text book. Thanks to all.

Charles Heick


#10

The post from Will Estevido gets at some interesting features of
gemstone hardness and toughness, but I think it raises more questions
than it answers.

What is meant by “resistance to change” and “cohesion”, for example,
in this context of hardness and toughness? I am asking for
definitions that will help us understand the concepts here.

Also of interest would be more details about “cleavage surface” and
how the number 111 was derived to denote the hardness of that surface
in diamond. What are the actual methods used to come up with this
"adjusted scale of hardness"?

You say, “When discussing hardness we are really talking about
breaking bonds between atoms and molecules. This can be a very complex
subject…” If you don’t mind Will, could you go on a bit here.

I didn’t know that jade has fibers that interlock. What I have seen
about jade suggests that jade is a composite material with a peculiar
arrangement of internal structure that gives rise to its inherent
toughness. The idea of fibers is a new one to me. Could you explain
more about that?

This is indeed a complex subject, and it needs to be discussed in a
more technically rigorous way. Otherwise, there is a tendency to have
more confusion than clarification. It appears that you have studied
this material in some detail. Would you mind sharing your references?

Thanks for your contributions.

Riccardo


#11

Hello Ron,Charles and all

Here is a copy of the addition I posted and.I do not know exactly the
difference in the crystal structure of the diamond.I wanted to point
to the fact that there is a difference in hardness beeing 9.5 or 9.6
or 9.845878965321 (whatever).However,some members didn’t receive the
message I quess.Perhaps I should of used the word “could be” or
"something like" instead of “can be” (here we go again !!!).

I do understand that diamonds are cut by diamonds relating to the
fact of the differents in crystal structure and imperfections -like
you explained very well Charles- and this is how they get cut using
this trick and knowledge.But this is the prove,diamond can cut
another diamond due to this difference in hardness how small it might
be and at that point you have an harder and softer material beeing
the same stone.

Try to scratch one of your fingernails with another one en let me
know how long it toke and the amount of scratches ?

I hope that you all understand that I’m NOT declairing myself as a
gemologist or mineralogist and I do not have any kind of education
other then what I’ve learned reading books and I do
respect (all of you by the way) your comments, but if you people are
wright well somebody has to be wrong and I’m relating to books.

Book 1: Color encyclopedia of gemstones (second edition) ISBN
0-412-98911-5.Be my gast and read page 14 of this book describing
hardness.Allow me without beeing procecuted to write a part of it. “If
the SECOND material is HARDER then the FIRST,it will leave …”.

Book 2: Gemstones of the world written by Prof.DR Walter Schumann.
This book is derived from the original one “Edelsteine und
schmucksteine” ISBN 90-5121-111-2. I have the dutch edition of this
book but let me translate it for you. The discription of hardness on
page 20 says "Any mineral of this serie (mohs scale)can be scratch by
the next higher one but not by the previous one.Minerals of the SAME
hardness CAN NOT scratch eachother.

By the way Will,compared the the scale of Rosiwal,diamond has a
hardness of 140,000,korund 1000 and topaz 175.Just some info I know
about to visualize the difference better between diamond,korund and
topaz.

Please do me a favour,read the book’s and then tell me if I’m wrong
or wright.Let me put it this way,tell me if they are wrong or wright
!!! Be so fair after reading those books to post an e-mail on this
forum to express your opinion.I feel not offended if one spreading
out better then mine and I do not have hard feelings if I
get corrected,that’s the purpose of having this forum … …or am
I wrong ?

Regards Pedro
Palonso@t-online.de


#12
Your statement that diamond can be 9 1/2 on the Moh's scale is dead
wrong. The actual hardness of diamond on an adjusted Moh's scale is
between 37 and  43 ! Furthermore, it is quite possible for minerals
of the same hardness to scratch one another. Indeed, how do you
suppose diamonds are cut? 

Perhaps a slight additional clarification will help those who aren’t
sure what all this means. First, as to the mohs numbers, there is no
accepted “adjusted” mohs scale. But the mohs’s scale is not a linear
scale at all, and if it were, then the numbers Ron gives would be
needed to put the appropriate space between corundum and diamond.
Just understand that those are not actual points on any mohs scale
anyone uses. The only use of the mohs scale is to rank minerals in
the order of which are harder than others. It’s not actually any
measurement of hardness beyond this. More accurate measurements are
with scales like Vickers, Rockwell, and the like.

Second, I think Pedro’s statement comes across as inaccurate only due
to language difficulties. Diamond does indeed vary in hardness. But
to be accurate, one should point out that the situation is NOT that
some diamonds are harder than others. What IS the case, is that the
hardness of diamond varies according to which direction through the
crystal you measure it. All of them do this. Scratching a diamond is
hardest if you attempt to scratch the octahedral faces of the crystal,
or in any plane parallel to where such faces would be if they were
there on that crystal. This is a defined plane within the atomic
structure of the crystal, and in that direction, bonds are densest
and strongest, and thus the diamond is hardest in those directions.
The Cubic crystal faces, on the other hand, are softest. In any
diamond crystal this is the same, with only very slight variations, if
any, from one crystal to another, and those due mostly to
impurities/flaws, etc. Diamonds can be cut because if you take a
powdered diamond grit, of random particles, and abrade a diamond with
it, in a direction other than parallel to an octahedral face, then
some of the particles of the grit will be exposing that hardest
direction, and these will then scratch/abrade the diamond crystal. If
you actually attempt to polish/grind the diamond on those octahedral
faces, you’ll get nowhere. It does eventually polish but only as fast
as the grit wears away the same amount. Like sanding wood with
sawdust. Eventually the wood will be polished, but it’s not the easy
way to do it. Polishers who find themselves needing to polish a
facet in one of those planes either choose to adjust the plane of the
facet a little bit so they’re not quite working in that direction, or
they’ll have to spend a very long time working that facet before it
responds at all, and even then, such facets generally don’t get a good
polish.

Peter Rowe


#13

Charles, Pedro, Peter, and others in this thread.

I guess what is important to understand here is that the forces and
materials needed to scratch any given surface are variable and, that
there are many hardness scales that that do not necessarily measure
the same phenomena. Take a look at
http://www.zeroonezero.com/uploads/tinius2/tmech.html to see a few. It
is possible to use conversion tables to compare measurements between
certain hardness scales, but as Peter pointed out, it is difficult to
try to compare or find equivalents between scales that differ in their
logic. (comparing apples to oranges sort of thing) … see:
http://www.mtu-net.ru/nanoscan/mater.htm for one of the latest
developments in diamond hardness testing. There is a small table
showing a comparison of a few What do you think about the
"hardness" for fullerite (C60)?

The (111) plane-- Imagine 3 lines, (axis,) crossing in space. Label
the line going from front to back as “a”, the line going from right to
left as “b”, and the vertical line going from top to bottom as “c”.
Lets call this set or coordinate system, (abc). Now if a plane (or
crystal face, or cleavage plane) is extended in all directions it must
intersect at least one of these lines. If a plane intersects the "a"
line only and not “b” or “c” at some unit 1, then we label that plane
as (abc) = (100), read as one- zero- zero, not one hundred. If the
plane intersects two lines say the “a” and “c” and not “b” then read
as (abc)=(101), one-zero-one. And if the plane intersects all three
lines at some unit 1, then read as (abc)=(111), one-one-one.

Jade --The fibrous nature of the jades does not mean loose fibers,
but instead a compact mass or aggregate of fine fibrous crystals. In
the case of nephrite, these are small curved and interlocked crystals
of actinolite, tremolite and a little epidote. And for jadeite, small
relatively straight short fibrous interlocking crystals of jadeite.

Regards, Will Estavillo


#14

Pedro, I do not know the variation in diamond hardness when
referencing the Moh’s scale, but 9.5 or even 9.8 would seem much too
low. Remember, the only way to permanently reduce the hardness (not
toughness) of a diamond is to replace the carbon atoms within its
crystal lattice with atoms of another element. In other words,
destroy or weaken its crystal structure or bonds. For example, when
Nitrogen atoms replace carbon, the diamond begins to yellow; Nitrogen
is what yellows a diamond. However, in a gem diamond, the quantity
of foreign atoms is so minuscule, its effect on hardness is
indiscernible.

Instead of scratching fingernails where you external forces are
limited, take two coins of equal value and scratch the edge of one
against the face of the other. The coins will no longer be in mint
condition.

Crystal planes are too complex to explain without illustrations, and
even then you have to use your imagination to fully visualize them.
This is an area I have difficulty myself in visualizing.

Books are great resources and are indispensable. Myself, I rely
heavily on reference books for I have difficulty remembering names
and terms if I don’t use them on a regular basis. Fortunately, in
most cases, I remember where to find the I have forgotten
over time. The problem I found is that authors are not always right.

As I wrote in a previous post and Ron also stated, materials of the
same hardness do scratch each other. This is an accepted fact in the
sciences. I could reference books, but others, including yourself,
would not be able to verify the without that specific
book in hand. There is no point in rewriting the book here.
Therefore, I have listed references on the Internet that are
accessible to all for both edification and verification.

Hardness Like materials do scratch others of like hardness:

Rock Shop & Rock & Gem Magazine, Article by Dr. William S. Cordua,
University of Wisconsin
http://www.rockhounds.com/rockshop/hardness1.html

Ganoksin, Article by: Charles Lewton-Brain 1986/94

The Mineral and Gemstone Kingdom, Article by: Hershel Friedman
http://www.minerals.net/resource/property/hardness.htm

The following definition is from the GIA diamond dictionary on
directional hardness: “Tendency for the hardness of a mineral to vary
with crystallographic direction. Hardness depends on the number and
strength of atomic bonds, which are usually greater within a crystal
plane than between planes. Diamond’s hardest polishing direction is
parallel to octahedral planes; the easiest is on the dodecahedral
face. A direction of intermediate hardness, which is also used in
polishing, is parallel to a cubic face.”

Crystallography Crystal systems:

Here is an excellent tutorial on “Crystallography and Mineral Crystal
Systems” http://www.rockhounds.com/rockshop/xtal/index.html

Charles Heick


#15

Dear Peter, Now you are taunting me to challenge YOUR statement to
the effect that Diamond hardness is not a variable except to the
extent that its hardness varies in accordance with crystallographic
orientation.On the contrary, the hardness of diamonds varies with
provenance ALSO. The hardness variation that I referred to is that
which occurs as a result of variations in accordance with origin.
Furthermore, scientists have discovered that synthetic diamonds can
be altered by changing the ratio of isotopes so that it is possible
to produce synthetic diamonds that are harder than naturals ! One of
the basic considerations that Pedro expostulated is that like
substances cannot abrade one another. I would challenge that
assumption by suggesting that many ocean beach gravels suggest the
opposite. Locally we have beach gravels that are a product of
alluvial deposition of rock and sand originating in the Coast Range
of Central California. The geology of the Coast Range is remarkably
uniform. It consists primarily of the Franciscan cherts which are the
product of subducted sedimentaries which have been metamorphosed into
Chert and Jasper.Subsequent sporadic volcanism modified these strata
by introducing hydrothermal mineralization which produced macro
crystalline quartz along with sparse deposits of Cinnabar and various
sulfides. A secondary constituent of the Coast Range consists of
extensive bodies of highly altered serpentine which occaisionally
contains minor amounts of Nephrite Jade in contact zones. Minor
bodies of rock include sandstones, shales, quartzite and marble.
There are no significant instances of mineralization which contain
minerals with a hardness in excess of seven. ( Garnet is the
commonest instance of a mineral which has a hardness in excess of
seven ) In view of the foregoing, it may be assumed that the beach
deposits of the Central California Coast are made up primarily of the
erosion products of the Coast Range; i.e. that no significant
presence of minerals with a hardness exceeding seven on the Moh’s
scale exists in the alluviam on the beaches. Herein lies the
conundrum; if, indeed, minerals of the same hardness do not abrade,
why is it that the gravels on the beaches have acquired a semi-polish
? Not only can one skip the initial shaping process in the tumbler,
the prepolish phase can be also be reduced. Since there is no
presence of a mineral harder than seven how is this possible ? Ron at
Mills Gem, Los Osos, CA. P.S. I still wonder why so many of us want
to hide behind the anonyimity of a url. Why not come out of the
closet and admit that we are actually individuals who have little to
hide and everything to gain by becoming recognizable individuals ?


#16

Hi Ron, If you think the study of the hardness of materials is
complex, just try to get into “polishing” as a topic. Polishing is not
simply a matter of abrasion but also includes burnishing, flow,
cavitation, and chemical attack. Try this, pickle a piece of sterling
wire, cut that in half. Now you have two pieces of material with the
same hardness. Rub them together to get a nice burnished finish. Now
do this with beach stones for a million years or so. Voila, polish
without grit. Add a little zircon, garnet and tourmaline to the mix of
quartz and magnetite sand and you can reduce most beach stones to
smaller beach stones in a short time (geological time). Most beach
stones have a large percentage of the feldspars which are softer than
the minerals in the sand. I could be wrong about this so I am looking
for someone to sponsor a trip for me to study the beach sands of
Tahiti :slight_smile: BTW most of the beach gravels on the California coast
originated far inland and in some instances much farther east than the
coast ranges. These were stream tumbled through many geological
environments, not just on the beach. Will Estavillo


#17

Dear Will, I certainly agree with you about polishing…it is such
an arcane technology that seemingly no logic applies. There are many
instances wherein you can cite the efficacy of polishing a mineral
substance with a substance that is less hard than the object being
polished. I have always been bothered by this apparent illogicity.
One of your statements DID bother me, however. Having lived on the
edge of the Pacific Ocean for many decades, I have spent a lot of
time beachcombing and collecting and have observed that virtually all
beach gravels and sand are of local and fairly recent origin. You
will recall that in San Diego County that many beaches have lost
virtually all of their sand. The consensus of opinion is that this is
attributable to two primary factors: one is that runoff through
natural river channels has been reduced by such things as dams,
concrete linngs and retention of water for domestic applications.
Another is the fact that breakwaters have broken down the normal
transport mechanism along the shorline. It is also fairly well
established that transport of sediments continues taking place after
they are deposited at the shorline. The natural tendency for
unconsolidated alluviam to obey the laws of gravity suggests that
ocean currents will continue to transport the alluviam to depressions
along the shoreline wherein they tend to move to progressively deeper
basins. The local origin of beach gravels is nowhere more evident than
along our beaches here in San Luis Obispo County. Not only can you
observe the relationship between shaping of the stones and their
proximity to estuaries, but you can also see that they are definitely
the product of in situ local rock strata. The local geology is very
distinctive and bears no relationship to rocks that might have had
their origins in distant locales. Ron at Mills Gem, Los Osos, CA


#18
   Dear Peter, Now you are taunting me to challenge YOUR statement
to the effect that Diamond hardness is not a variable except to the
extent that its hardness varies in accordance with crystallographic
orientation.On the contrary, the hardness of diamonds varies with
provenance ALSO. 

Ron, I won’t go back and reread my post to be sure, but I think I
included the comment that diamond’s hardness does indeed vary SLIGHTLY
due to differences in composition, impurities, etc. This is, of
course, what you’re referring too. However, it should be pointed out
that these variances in hardness due to provenance, or differences
from stone to stone just due to impurities,inclusions, etc, are quite
minor and secondary to the differences in hardness due to
crystallographic orientation. At least, that is what I’ve been led to
believe. If I’m wrong here, and the provenence/etc variations in
hardness are indeed substantial enough to equal or exceed those of
orientation, please correct me, with appropriate references to the
published literature so I can verify these figures and update my
My current understanding is simply that the purest
diamond crystals, with the fewest impurities and the fewest structural
defects, will be the hardest. Is this correct?

And, as to items of like hardness abrading each other, this is a
silly argument we’re all having. Abrasion is not an all or nothing
action. Water is soft, yet look what it manages to carve out of the
much harder rock all around us. quartz does not abrade quartz with
the ease that corundum abrades quartz, but it most certainly does
abrade it. Hell, given enough time, and enough talc, you can abrade
quartz with talc. It’s just that you’ll go through a hell of a lot
of talc before you make that mark on the quartz. But make it you
will. I never said you could not abrade diamond with diamond, even at
the same hardness. If this were the case, then facets placed on
rhombohedral crystal faces could not be polished. They do get
polished, showing that it’s possible. Likewise, knots in a diamond
crystal presenting a hard direction to a diamond saw, do eventually
get sawn through. But it’s slow as hell, taking many many times as
long as it should otherwise have done, possibly even ruining several
diamond saw blades, and annoys the diamond cutter a whole lot. So
while in theory you can abrade a substance with an abrasive of the
same hardness, IN PRACTICE, it ISN’T done if it can be avoided. Takes
too long. When Palonso says you can’t do it, then perhaps he’s just
speaking practically rather than splitting theoretical hairs. In the
business of diamond cutting, you shouldn’t try to do it if you can
avoid it. Costs too much. But if you, sir, wish to try it just to
proove to yourself or us that it CAN be done, please feel free. I
agree with you that you’ll eventually succeed.

Peter Rowe


#19
   "Water is soft, yet look what it manages to carve out of the
much harder rock all around us." 

Peter, other than the brut force of water moving rock, isn’t it the
impurities and material being carried by the water that is actually
doing the carving and not the water itself? And if so, wouldn’t this
be more and result of a materials toughness than hardness? That is
to say, like trying to damage a diamond with a blow from a hammer
(toughness) as a distinction to scratching (hardness) one.

Charles Heick


#20

Ron, I hope we are not getting too far off topic with this discussion
about beach gravels. Perhaps we can justify this bandwidth by saying
that many gemstones such as diamond, sapphire, ruby, zircon, etc. are
found in gravels and the geological processes we are discussing does
have some direct importance to jewelers. Certainly many stones of the
first water are naturally cobbed in streams and beaches and can have a
higher value than those mined in-situ. Back to west coast USA, The
California coast has approximately 59% sand beaches, 38% rocky shores,
and about 2% gravel or cobble beaches. The vast majority of cobbles
found in Southern California beaches originated from conglomerates
such as the Poway Cong. which in turn had a source inland many miles
from the present shoreline. The gravels from North Central and
Northern California beaches have a complex history. Certainly many
concentrations of gravels there are derived from local sources, but
these can be mixed with stones derived from m?ange conglomerates, and
your guess is as good as mine as to what pre- subduction terrain these
came from. Check out http://www.sonoma.edu/geology/wright/shellb.html
for a better description.

BTW I just read that Morro Bay is filling with sediments and at the
current (no pun intended) accelerating rate, should be full of sand
and mud in about 300 years. I did a show there in the late 70’s,
Didn’t sell much jewelry, but found a beautiful piece of nephrite on
the beach to the north of one of the Nine Sisters. Will Estavillo