Administered by Hale Sweeny (firstname.lastname@example.org)
This list digest contains the following message subjects:
1. LapDigest News for Issue No. 44 Tuesday 8/12/97
2. NEW: THE HARDNESS OF MINERALS AND ROCKS - PART I
3. NEW: Hardness vs Toughness
4. RE: Tiger-eye Treatment (Was 13-1 and 26-3)
5. TIP: Polishing Speed
6. Re: Drilling Fragile Stones (Ultrasonic Drills)
Subject: LapDigest News for Issue No. 44 Tuesday 8/12/97
We are happy to present the first of a three part paper on
hardness, one of the most important of the physical
properties of rocks and minerals. The second part will be
published in the next issue and the third in the succeeding
issue. This paper was written for the Digest by Dr. William
Cordua, Professor of Geology at the Univ. of Wisconsin at
River Bend. Along with the third part, I will present
information on several commercially available kits for
measuring hardness, where to buy them, their present prices,
and so on.
Why should a lapidary be interested in measuring hardness
of the material being worked on? There are many reasons,
but here are two: first, it is an important clue to the
identification of the material. Next, it gives some
information about the 'polishablilty' of the material.
Hope you enjoy this paper.
Subject: NEW: THE HARDNESS OF MINERALS AND ROCKS - PART I
Hardness tests of minerals are among the easiest
and most useful tests to perform. What rockhounds speak of
as hardness is more accurately described as resistance to
abrasion. We are testing how easily one substance will
scratch another. As an example, copper is relatively easy
to scratch, but would you bet on diamond or copper standing
up better to blows from a hammer? Hammer blows measure the
ease with which something fractures or its tenacity. There
are other hardness scales than are based on ease of
indentation, resistance to twisting and so forth. For the
sake of simplicity and standard usage, in these articles,
hardness will refer to the resistance to abrasion as given
by Mohs' Scale.
The classic scale for hardness was published in
1822 by Frederick Mohs, an Austrian mineralogist who got
the basic concept from scratch tests performed routinely by
miners. Since Mohs published the scale, it bears his name
rather than that of the unknown genius who thought of it.
The scale selects 10 minerals as standards, arranging in
order of increasing hardness. These are, as most of you
1 = Talc
2 = Gypsum
3 = Calcite
4 = Fluorite
5 = Apatite (fluorapatite)
6 = Orthoclase
7 = Quartz
8 = Topaz
9 = Corundum
10 = Diamond
These minerals were selected for their abundance,
as well as their differing hardness. The scale is uneven.
For example. diamond at 10 is much harder then corundum at
9, while fluorite at 4 is only slightly higher than calcite
A more limited but practical scale can be easily
and cheaply obtained by observing that
....your fingernail has a hardness of 2.5,
....a penny has a hardness of about 3.5,
....glass and a steel nail have nearly equal hardnesses
of 5.5 and
....a streak plate has a hardness of 6.5.
If I carry a nail and streak plate with me and can scrounge
up a penny, I've got a handy, light weight mineral testing
lab in my pocket.
More expensive sets can be bought. A set with
small samples of all of Mohs' minerals allows a bit more
precision in testing. The specimens do lose their
usefulness the more they are scratched up in various tests.
As an alternative, one can custom build their own Mohs set
through collecting or purchasing small fragments of the
needed minerals. Other venders provide sets of hardness
pencils with tips of two natural or artificial substances
of measured hardness. These are handy in that they are very
precise and allow one to test a small surface easily.
Most mineralogy texts give tables of mineral
hardness. Particularly complete and useful tables appear in
John Sinkankas' "Gemstone and Mineral Data Book."
Dr. Bill Cordua, University of Wisconsin-River Falls
-- non-commercial republish permission granted --
Subject: NEW: Hardness vs Toughness
Just got back from the Bancroft Gemboree. I went with a
friend with whom I had a discussion about the hardness of
minerals vs. the toughness of minerals, and how this can
impact how a stone is cut/polished. The example discussed
was rhodonite, which is apparently a "tough" mineral to
1) What's the difference?
2) Could someone give examples of minerals with the same
hardness but different toughness to illustrate?
Toronto, Ontario, Canada
(Ed. Note: To start this discussion, toughness is defined
in Sinkankas' Mineralogy (p160) as the degree of difficulty
with which a material is fractured. He says that strength
may be demonstrated by fracturing specimens with a small
hammer, and notes that very brittle materials include
opal, sulfur, halite, calcite and galena, the last three
serving to show how easily cleavage is developed. Another
brittle material, which illustrates conchoidal fracturing,
is obsidian. "Tough minerals include corundum, chalcedony,
and, outstandingly, rhodonite,nephrite and jadeite which
can only be broken with considerable difficulty"(sic)
His definition does not directly apply to difficulty to
polish, but is a physical strength property. Since many
physical properties are interdependent, it is possible that
toughness and difficulty to polish are correlated. Dianne
has already noted that rhodonite is hard to polish, and is
one of the three toughest minerals cited above. Many of my
friends find jade very difficult to polish, and it's the
other two of the toughest three! This would make it three
for three. But I wait for others more knowledgeable than I
to answer this! hale)
Subject: RE Tiger-eye Treatment (Was 13-1 and 26-3)
(Ed. Note: I really hate it when a query goes unanswered.
One was posed back in Issue 13 (June 29th) by Dan
<email@example.com>, and I just saw the answer in a Tip in
the latest AFMS Newsletter. Dan asked:
<I've seen nice catseyes produced from tiger-eye. I
understand they use acid to lighten the tigereye to the
light honey color. Oxalic acid might be the acid. Is
someone familiar with the exact process?>)
OXALIC ACID LIGHTENS TIGER EYE as well as cleaning iron
oxide off quartz and the like. Use 1 tablespoon per cup of
water. OXALIC ACID IS A POISON, so do NOT use cooking
utensials if you decide to boil to speed up the reaction.
A bit of aluminun foil or nail will speed things up. Green
water means it's working. It's best to work with a
finished, but unpolished cab.
Petrograph via Breccia et. al.
AFMS Newsletter 9/97 Part 1
Subject: TIP: Polishing Speed
From: AFMS Newsletter 9/97
POLISHING SPEED can be changed by choosing the proper
particle size polish. Fastest would be 1.0 micron such as
Linde C. Intermediate would be 0.5 micron such as Linde B.
Slowest would be 0.3 micron such as Linde A. Of course, the
"shineyness" of the polish goes the opposite of speed. So
you might wish to speedily polish with 1. or 0.5 and then
finish off with 0.3 to get that extra shine.
Subject: Re: Drilling Fragile Stones (Ultrasonic Drills)
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
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