Gem Hardness

Jewelry Discussion
1 
   as a pragmatic lapidary & designer i look for the least
complicated way to attain the results i need. for drilling pieces
of quartz, the next notch down from diamond on the mohs hardness
scale, i

Huh? I normally let minor inaccuracies pass but this one has to be
pointed out. That “notch” leaves out a few rather important
minerals. Diamond is Mohs 10, and it’s not a proportional scale.
Diamond is many times harder (in some crystal directions!) than the
next-hardest mineral, corundum (sapphire and ruby) at 9. Then
there’s chrysoberyl (alexandrite, cat’s-eye and golden and green
chrysoberyl) at 8.5. YAG (a man-made “garnet”) is 8.25. Next in line
are spinel, topaz, taaffeite, rhodozite. phenakite and some natural
and synthetic beryls at 8. The 7.5 “notch” is occupied by some
beryls, and zircon, almandine garnet, hambergite, euclase, gahnospinel
and gahnite. Several members of the garnet group (rhodolite, pyrope,
spessartite) range between 7 and 7.5, as do tourmaline, andalusite,
iolite, and staurolite.

Now we’re down to quartz, which shares the 7 position with grossular
garnet, danburite, dumortierite and occasional chalcedonies,
peridots, jadeites, andradite garnets, axinite, and saussurite.
Some quartz minerals like chalcedony, agate and jasper can range from
6.5 to 7 and a little above.

I agree with your solution to the drilling problem, although if a lot
of drilling is required it would be better to contract the job out to
someone with modern sonic drilling equipment. Back when I started in
lapidary work (dinosaur coprolites were not yet fossilized!), we did a
very satisfactory job of drilling quartz using mild steel or brass
drill bits with loose silicon carbide grits for abrasives (all
underwater of course). Diamond bits, especially at current prices,
represent an order of magnitude technological advance and are a
bargain to boot.

I’ve seen beautiful examples of drilled quartzes accomplished by
pre-Columbian societies in South America. The work was probably done
with bow drills and alluvial garnet, beryl or corundum sands as
abrasives. Such skills have existed for millenia. The Romans traded
diamond-tipped drill bits over 2,000 years ago! The diamonds probably
came over the Silk Road from the fabled Golconda mines in India. The
ancients were very quick to observe the physical properties of various
minerals and how they could be turned to an advantage in commerce.

Sorry for the lecture.

Rick Martin

2

Rick Martin has done his usual great job of expounding on a
gemological subject and I would like to add a bit to
it…Diamond is actually between 38 and 43 if you were to put it
into gemological perspective relative to the Moh’s scale of hardness,
The Moh’s scale sucks. It is an archaic holdover that should have
been supplanted long ago ! Lecturing prospective Diamond buyers with
a spiel on the true hardness of Diamond is a great way to convince
them of the superior wear characteristics of the stone. Ron at Mills
Gem, Los Osos, CA.

3

And remember that the Mohs scale is not even close to being linear
(the “distance” between steps/notches is not at all the same between
the stones on the mohs scale) . I terms of true measured hardness,
and depending on how you test the hardness, diamond can be some 40 or
more times harder than the next stone on the mohs list, corundum.
Even then, the hardness of diamond varies a good deal depending on
which crystal face you measure. Cubic crystal faces are softest,
octahedral faces are the hardest (that variability if why diamond
dust can be used to cut and polish diamond at all).
Peter Rowe