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[How2] Testing toughness of gemstones


Have any of you ever seen a description of how testing for
toughness of gemstones is done? I don’t remember seeing it in
any course material. John


I don’t know how to test for toughness, but the issue (and many
other cutting characteristics) is addressed for most stones in
John Sinkankas’ Gem Cutting; A Lapidary’s Manual.

John McLaughlin
Glendale, Arizona


John - My recollection from mineralogy is that toughness is
determined by percussion. Briefly, a sample of a substance is
ground and the top surface polished. It is then inserted into a
gadget which has a heavy stylus attached to a vertical support;
the stylus is tipped with a diamond point. The stylus is raised
to a certain height, and released. The toughness of the material
being tested is determined by the depth to which the diamond
point penetrates it. Might be mistaken, but that’s what I
remember (undergrad days in the mid 60s).



Start with a small light hammer, and hit the stone very gently.
Increase both the vigor of the impacts and the weight of the
hammer until the stone shows damage. Record your results…

(just joking)

Normal gem testing does not include testing for toughness. It’s
not a precisely measured or scaled quantity, since it’s a
combination of many factors, including hardness, elasticity,
tensile strength, etc. These are standard engineering tests, and
the overall combinations will tell you roughly how tough a
material is. But again, these aren’t generally tests performed
on individual stones as part of gemological practice. Much of
the info on gem toughness is as much as anything, anecdotal.
Need to know how tough a given variety of stone is? Ask the
setters who set the things…

One reason why toughness testing isn’t especually useful is that
even within a given type of gem, it can vary greatly from sample
to sample. Inclusions, residual stresses from cutting, heat
treatment or natural heating, imperfect crystalization, and all
the rest, can greatly affect how easily a stone can break, even
if it’s hardness remains the same. some parts of an emerald
near a feather or a large bunch of inclusions will be less tough
than other parts of the same stone without these stresses and
weak points. So toughness is a useful think to know, as general
knowledge in choosing stones suitible for a given piece of
jewelry, or choosing setting and handling methods for a given
stone. but beyond that, it’s too variable to be of much use.

Peter Rowe


I do not think testing for toughness is possible. There are too
many variables. Hardness, cut, cleavage, flaws, etc. Generally
experience is the best gauge. I have repaired many corundums
(sapphires, rubies) with totally scratched crowns (tops).
Corundum is normally thought of as one of the more durable
stones. What harms gemstones are sharp edges and impacts. For
instance a large star sapphire in a mans ring that is banged
against metal or other objects will chip. Also facetted stones
that are not protected will easliy chip. Impact is what does
it. The object the stone impacts against does not have to be as
hard as the stone. Impact plus the sharp edge chips the stone.
Therfore a tough stone ends up chipped.

Gerry Galarneau


Hello John, During the California Gold Rush, the story goes,
miners would put a suspected diamond on an anvil and strike it
with a hammer. If the stone remained intact, it would then be a
diamond. I wonder how many diamonds were converted to grit this
way? Hardness is defined as the resistance to abrasion
(resistance to scratching). Toughness is not a quantifiable term
but more of a descriptive term of the resistance to “breaking”.
The quantifiable aspects of toughness come under the heading of
Tenacity and include the terms: Brittle, Malleable, Sectile,
Ductile, Flexible, and Elastic. Shear, stress and strain are the
mathematical terms used in measuring the above. Some materials
can be both very hard and very brittle (emerald). And other
materials can be both soft and tough (jade). It is good to know
these properties while creating jewelry, but it is not good to
test a finished piece (or a suspected diamond crystal) with
hammer and anvil. Hope this helps, Will Estavillo >


John, Are you asking about the Mohs scale of hardness? That
should appear in any earth science of geology text, What do you
mean by toughness? Steve Ramsdell