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This list digest contains the following message subjects:

1. LapDigest News for Issue No. 45
3. RE: Hardness vs Toughness
4. RE: Hardness vs Toughness
5. NEW: Sphere Machine
6. NEW: Polishing & Dyeing Geodes & Agates
7. RE: Tiger-eye Treatment
8. RE: Drilling Fragile Stones
9. RE: Drilling Fragile Stones (Ultrasonic Drills)


Subject: LapDigest News for Issue No. 45

If you have questions about physical properties of the
stones you work with, please send them in while we are on
this topic.


Doing hardness tests requires some technique. You
need to find a good surface or edge on your unknown to test.
Take care to make sure you are testing the right grain - not
the bit of quartz right next to it. In some case it is
easier to scratch the unknown across the standard. (the
point of a unknown mineral grain across a calcite cleavage).
In other cases it is easier to test the standard across the
unknown ( tip of a nail across cleavage surface of the
unknown grain). In an ideal case, you should try to do
both, to double check your findings. You need to press
hard enough to good effect, but not so hard as to fracture
either sample. Practice will help you get the proper level
of stress to exert.

As a result of your test, you will look for a
scratch. Rub aside any powder to see if a distinct scratch
has been left. Calcite will leave a trail of powder across
quartz. Rub away the powder and you'll see the quartz is
unharmed. A hand lens will help you see the scratch. In this
way you can bracket the hardness of your unknown between two
of your standards (harder than a fingernail, softer than a
penny). The ease with which one substance scratches another
is also useful. Quartz easily scratches calcite, telling you
of a large hardness difference. Quartz will scratch feldspar
with much more difficulty. When testing a standard against
an unknown that is of equal hardness, both substances will
leave shallow scratches on each other.

The hardness of a particular mineral may vary with
direction within the same grain. Kyanite is a good example.
Kyanite generally occurs in long bladed crystals. The
hardness taken the short way across the blade has a hardness
of 7 the hardness taken the long way along the same grain
will be 4.0. Muscovite is another good example of this. Its
hardness is 2.5 when taken across a the flat surface of a
cleavage sheet, but 4 when taken across the grain of a book.

The reason hardness varies in this way is that the
phenomenon depends on the strength of the bonds holding the
mineral together. The bond strength can be significantly
different in different directions in the mineral, giving the
different hardness. In most minerals this difference with
direction is minor and doesn't affect the test. In the case
of kyanite, this difference in hardness is a confirming test
by itself.

Some minerals' hardness may vary from sample to
sample depending on that mineral's exact chemical
composition. Hornblende's hardness can vary from 5 to 6,
meaning some hornblende is softer than glass, some harder.
This reflects the fact that hornblende can accommodate
varying amounts of sodium, calcium, iron and magnesium in
its structure, which affect the details of its chemical
bonding, hence its hardness.

(Part three will be published tomorrow)

Dr. Bill Cordua, University of Wisconsin-River Falls

-- non-commercial republish permission granted --

Subject: RE: Hardness vs Toughness

Hardness vs. Toughness:

>My questions:
>1) What's the difference?
>2) Could someone give examples of minerals with the same
>hardness but different toughness to illustrate?

There's a famous story about a diamond merchant during the
early days of diamond mining in South Africa. I think
Eric Bruton tells it in his wonderful treatise, "Diamonds."
Diamonds are the hardest known substance in the universe.
This swindler invited prospectors to test diamond "hardness"
on his anvil: if the diamonds shattered when hit with a
heavy hammer they weren't diamond (he waited until the
suckers were well gone then hunkered on his hands and knees
and picked up the shards. He made a good living knowing
the difference between hardness and toughness. And in the
process he allowed a 50-plus carat red diamond, probably
the most valuable stone ever discovered in human history,
to be destroyed).

Despite its awesome hardness (in some directions), diamond
(like lots of other crystalline minerals) has several planes
of serious weakness. You've all seen the TV commercial of
the guy in the back seat of the luxury sedan whose heart
nearly stops when he cleaves a big diamond. But how well
known is the fact that there are directions on a diamond XL
that can't be sawn even with a diamond saw? Or any other
known means?

As a jeweler I can't tell you how many chipped diamonds I
see. When I take a stone in for repair the first thing I do
is clean it and check for owner-inflicted damage. The best
thing a jeweler has going is the old wives' tale that
diamonds can cut glass. Those who try it often end up
having to replace a diamond because glass can also cleave
or chip diamonds.

The difference between hardness and toughness in a mineral
has to do with internal structure. Diamond crystallizes
in the cubic system. The atomic lattices that create cubic
XLS have planes of weakness along the XL faces. So diamonds
and fluorite XLS, to name a couple, can be easily cleaved
in those directions. Other examples of XLS with differing
directions of hardness/cleavage in other XL systems are
topaz, mica, the feldspar minerals, etc. Amorphous
cryptocrystalline (hidden or minute XL) minerals like
chalcedony and agate have respectable levels of hardness
but are brittle. The jades and rhodonite are
cryptocrystalline too and have about the same hardness,
but have an interlocking XL structure that imparts much
more resistance to flexing and impact.

I know this is brief and incomplete but I hope it helps.
For those who would understand (and sell) gems and minerals,
this is one of the crucial areas of product knowledge.

Rick Martin

Subject: RE: Hardness vs Toughness

Long time ago, I read that jades were so tough that you
could throw a cab on a concrete floor and it would NOT
break. Not long after that I cut a Wyoming nephrite -
about 15mm. Polished OK but the color wasn't good enough
to keep. So I gave it the test. Thing bounced way up --
in two pieces. Some things aren't as tough as they're made
out to be, I guess.

Bob Foster

Subject: NEW: Sphere Machine

Anyone know where I can get a set of plans to make a
home-brew sphere machine?

Subject: NEW: Polishing & Dyeing Geodes & Agates

I love the simple process of cutting geodes and
agates. (I am just a beginer, can you tell? lol) I have two

....I have a 14" vibrating lap polisher for large pieces,
but there must be a much faster way to polish small geodes
etc... Would anybody tell me how?

....I see all kinds of colored geodes, etc. in the stores
that have obviously been dyed. How do they do that?

Thanks for your help.
--Non-commercial republish permission granted--

Subject: RE Tiger-eye Treatment

This query had to do with bleaching tiger's-eye to make
perfect cat's-eyes. Yes, it can be done, and I've had
thoughts on this since the question was posed. Here're
my deux centimes. I've tried two or three different
approaches to this using dangerous chemicals over the years
and had differing degrees of success, exposing myself and
my family to undue risk in the process. (I haven't tried
oxalic acid, but I've experimented with hydrochloric, some
"serious" chlorine bleaches and other stuff). But the
all-knowing experts in Idar-Oberstein perfected this trick
in the last century and are willing to sell their creations
for literally pennies. Why bother with the alchemy and the
danger? Yes, I like to experiment too. But if anyone wants
to buy such cabs and doesn't have a source (and wants to
avoid the serious chemical exposure) I'll be happy to do
some research and find someone who sells them reasonably.
Maybe prices have changed since I was last in this market
but I doubt the change has been that great.

Rick Martin

Subject: RE: Drilling Fragile Stones

At 10:34 AM 8/10/97 -0700, you wrote:

<<I have a friend .. (snip) .. who is a dealer in rare gems
... He is having trouble drilling small and fragile pieces
by conventional means & asked me to post a request for
information about ultrasonic drills (or other suggestions).
Cost is important- ..(snip).. ..any recommendations would be
appreciated. >>

I think you might find ultrasonic is to violent for your
needs. Look into lasers.If you like I will try to get you
contacts in New York for contract laser firms.They mainly do
Diamond and Emerald, but would be the best to discuss your

(Hey I tried to reach you for some time to find out more
about the OTHER Amethest mine there, get back to me with a
good E-mail, it kept bouncing.)

Mark Liccini

Gemstone Rough Dealers since 1970 U.S.MAIL
E-Mail: 107 C.Columbus Dr.#1A Jersey City,N.J.07302
Voice Mail/Fax: 201-333-6332

Subject: RE: Drilling Fragile Stones (Ultrasonic Drills)

In Issue 44, you wrote:

<<I am interested in info about the cost and source for
ultrasonic drills. Also if anyone has experience with them
I would like to know how well they work, how fast they
drill, etc.>>

Imahashi is the largest manufacturer of them. You'll find
all such manufacturers in the Thomas Register (URL is

Mark Liccini
Gemstone Rough Dealers since 1970 U.S.MAIL
E-Mail: 107 C.Columbus Dr.#1A Jersey City,N.J.07302
Voice Mail/Fax: 201-333-6332
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