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In praise of Mohs' scale


#1

Lots of good stuff recently on Mohs’ scale, and special thanks to
Charles for the comparison with Sclerometer values.

I’d just like to point out why Mohs’ was brought into being. It had
nothing to do with lapidary work. Rather it was intended as an aid
to mineral identification and description in the field or in a less
than ideal laboratory setting. Useful diagnostic features include
colour, streak, density, fracture and relative hardness. An unknown
mineral can be placed on the scale by physically determining which of
the reference minerals are scratched by it and which it in turn is
scratched by. Used as such the scale is extremely useful to
geologists.

When talking about what we may call “real” hardness measurements it
is vital to mention also the method used, as different techniques give
different values, and there is not always a unique conversion factor.
Hardness as a property does not have a real firm definition that
enables unambiguous absolute measurement. – Kevin (NW England, UK)


#2

Dear Kevin,

Your dissertation on hardness from the perspective of the
geologist/mineralogist is well stated,but when it comes to jewelry
there is much more to it than mere identification. When a person
spends a considerable sum on a faceted stone and that stone becomes
relatively worthless within short period of time, you are going to
have a very serious customer relations problem. While it is not a
valuable stone, cubic zirconia is an outstanding example of how a
stone with a high hardness rating can exhibit severe wear damage in a
short space of time. When the hucksters tout it as being “close” to
diamond on the hardness scale they are skirting the issue by not
revealing the fact that the Moh’s scale is ridiculously misleading.
The latest issue of Lapidary Journal has an article on the subject of
stone durability in jewelry applications. (page 19, August 2000) It is
a nice attempt to expound on the subject, but it may be that it was
compiled by someone who is more into writing than jewelry making . For
example it states that Jade is a top choice for durability because of
its toughness. True enough, but it fails to address the fact that Jade
is relatively soft and will abrade quite easily. Imperial Jade can
sell for a thousand dollars per carat…you would be ill advised to
use it in an unprotected design. The article goes on to say, under the
heading: Best Choices…“Quartz-Agate, amethyst, black onyx,
citrine,chalcedony, jasper, smokey quartz, tigereye…anything in the
quartz family will take the beating to which rings are subject.” Now
this is pure and simple mis!!! The most common
abrasive substance in most any environment is sand, which is
essentially quartz. And, as any one who is a lapidary knows, like
substances will abrade one another. Furthermore, there is a vast
difference between the micro crystalline and macro crystalline forms
of quartz when it comes to brittleness. When was the last time you set
a small marquise cut amethyst in a six prong setting ? If you are in a
hurry with this task you had better damn well have a spare stone on
hand ! Under the heading “Stones that need care” the author makes this
statement…“Topaz-Hard, but brittle. Good for occaisional wear,
dubious for everyday wear.” WHAT A CROCK! True, topaz has a well
defined basal cleavage, but the author has obviously never seen the
river rolled cobbles of topaz which are common in some areas of the
gem pegmatites in Brazil. Topaz has to be damned tough to not be
obliterated by miles of torture by rolling on the bottom of a fast
moving river ! Furthermore, in thirty two years of setting stones I
have never broken a topaz when setting. Now if it were a shallow
marquise Imperial topaz you can be sure that I would excercise caution
in setting…just as I would with a diamond of that cut. (Another
good example of a stone that is exceedingly tough, but has a strong
cleavage)

Ron at Mills Gem, Los Osos,
CA.


#3

Reply to Ron at MillsGem

Your expertise in stones is evident and there was much in your post
about stone durability that was useful.

Perhaps you would like to make a stab at a stone durability rating,
informal though it may be, that would give us your opinion of the
practical durability of various stones. It would be good if you were
to include reference to toughness, as well as hardness, in the same
rating.

Personally, I would be interested in your comments about the harder
stones like corundum, beryl, spinel, and chrysoberyl.

One minor quibble about your comments is that I get the impression
that you are of the opinion that stones which can’t hold a high polish
have demerits attached to them due to that. I get that impression
from your comments about jade in particular. If true, I think this
reveals a bias on your part that stones must be able to keep a high
polish in order to be judged durable. I would disagree with that
opinion, at least in certain cases, and here’s why.

I saw a client wearing a jade wedding band that had been on his finger
for some 25 years or so. The finish, by wearing it steadily all that
time, was in my opinion aesthetically insuperable, aside from the fact
(and maybe because of the fact) that it was very finely abraded all
about. The finish that the ring had acquired was intimately his, and
the softness of that finish was really beautiful.

So in this case, the toughness of jade was amply demonstrated, and the
fact that it was subject to abrasion is not a defect, but in fact an
asset, at least in my opinion.

As long as we are on the topic of surface finish, allow me to extend
the point to finish on metal as well. Surface finish can vary from
matte to mirror, and there is no intrinsic reason to value one over
the other. Finish is an aesthetic choice basically.

Even so, I notice a predilection toward high polish in the great
majority of cases in this culture at this time. It wasn’t always so,
and it doesn’t necessarily have to be that way now. I would like to
remind people to be sensitive about surface and to think about the
range of choices that are available before blindly committing to the
highest polish that they can achieve.

By the way, I am aware that high polish results in more sales. Even
so, in my view that is not a compelling reason to use it all the time.

Riccardo


#4

Dear Riccardo, Your query and comments about durability, hardness,
toughness and polish are well reasoned and eloquently stated. If I
were to expound on the subject to the extent that you have exposed
it, would take the better part of a day so I am going to try to float
some generalizations that will apply in accordance with my
experience:

  1. A stone can be durable,but easily abraded. ( Jade, agate etc. )

  2. A stone can be hard, but fragile. ( Emerald, Kunzite, etc.)

  3. All stones are very soft compared to diamonds.

  4. Faceted stones are more easily damaged than cabochons.

  5. Loss of polish greatly lowers the optical characteristics of all
    stones.

  6. The ambient abrasion environment of the wearer greatly affects
    stone durability. ( Some regions have a high incidence of natural
    abrasive materials and some occupations are conducted in abrasive
    environments i.e. painters-sandpaper, machinists-grinding wheels) 7.
    The lifesytyle of the wearer is critical to the survival of the gem.
    Some ladies will garden wearing their delicate cocktail rings, many
    ladies will sleep in their jewelry, some ladies will wear opal rings
    while showering in hot water. Furthermore, most younger ladies and men
    are very active and expose their jewelry to extreme situations (
    mountain biking, car repair, clumsy use of the hands, etc.)

It behooves me to sum up my generalizations by saying that it is
incumbent upon the jeweler to educate the consumer on proper use of
his or her jewelry. You don’t have to belabor the issues…just
impress them with the need to respect their purchases by treating
them as if they were important parts of their lives that need care
and protection just as children do. Later, Ron at Mills Gem, Los
Osos, CA.


#5

Ron, at last something we agree on. I wholeheartedly support your
reply to Riccardo regarding durability of gem materials. Well said!

Daniel R. Spirer, GG
Spirer Somes Jewelers
1794 Massachusetts Ave.
Cambridge, MA 02140
@spirersomes
http://www.spirersomes.com


#6

Ron, sorry to be so late responding to this post from 25 July.
Somehow it slipped through the net. I agree entirely with everything
you say, which actually is totally in line with my earlier post
(dissertation? … you flatterer !).

Just to restate what I said earlier. Mohs’ scale was devised as an
aid to mineral identification in the field or as an adjunct to lab
tests. It says nothing about durability under certain conditions. It
does not address anything other than what it sets out to do, which is
to establish a number of reference minerals, so that an unknown can be
ranked as “scratched by this one but not by the next lower one” with
reference then being made to a table of relative Mohs values. Most
minerals of potential interest to geologists have absolutely no
lapidary/jewellery value at all. Really it’s the old comparing apples
and oranges thing. If dealers out there are using Mohs’ scale to pass
something off as having certain durability properties then firstly we
shouldn’t let ourselves be taken in by it, and secondly we should try
to put a stop to it. Your use of the term “huckster” is noted, and
supported. Lets none of us be taken in.