Edited and Published by Hale Sweeny
Web Site:
Associate Editors: Geo. Butts, JR Shroeder, Steve Henegar
and Margaret Malm

This list digest contains the following message subjects:

1. LapDigest News for Issue No. 225 - Fri 8/13/1999
2. New: Mineral Oil as a Trim Saw Coolant
3. New: Filling Cracks in Dino Slabs
4. NEW: Star Garnets
5. RE: Rainbow Obsidian Structure and Colors
6. RE: Rainbow Obsidian Structure and Colors
7. RE: Rainbow Obsidian Structure and Colors
8. RE: Rainbow Obsidian Structure and Colors
9. RE: Rainbow Obsidian Structure and Colors
10. RE: Rainbow Obsidian Structure and Colors
11. RE: Parts for 6" Sears and Roebuck Gem Maker
12. RE: Does Alunite Need to be Stabilized?
13. RE: Does Alunite Need to be Stabilized?
14. BIO: Alicia Carr
15. WTB: "Yellow Cat" (Redwood) Petrified Wood
16. FS: Obsidian Chunk


Subject: LapDigest News for Issue No. 225 - Fri 8/13/1999

Other than Gem clubs, there are several societies relating
to Lapidary. An example is the American Opal Society. I am
also aware that there is an agate society. If you know of
any such societies, please send their name and address. I
want to prepare a list of such groups for the Archives.

Went to the EFMLS show and convention this past weekend in
Washington, and then spent several days with my daughter
and grandchildren in NJ. That is why the Digest is late!!

Take care


Subject: New: Mineral Oil as a Trim Saw Coolant

Hi, I have a question on saw blade coolant. I got an old
Frantom 10" trim saw a while back and started looking for
something more user friendly than the diesel/crank case oil
that was in it. I reviewed you FAQ and didn't see anything
on using plain old mineral oil from the drug store.

An old timer told me about it. No rust (yet), the blade
hasn't glazed up and best of all, no smell from the spray.
Am I setting myself up for some problems in the future?

Thanks for any help.

Jerry Kinder
Jerry: No problems that I know of. You might want to
compare the price you pay for mineral oil against the price
you would pay for the same quantity of a saw oil like Almag
or that other one - whose name escapes me for the moment.
You might find that Almag, bought by the gallon, would be
cheaper! hale

Subject: New: Filling Cracks in Dino Slabs

I picked up some dinosaur bone slabs, and some are porous
and some fractured (the only ones I could afford). I plan
to make some cabs for bolo's out of them, but am a little
worried about them falling apart before I get a polish on
them. Is there a way to fill the cracks before I shape
them? I have thought of soaking them in glue, maybe
superglue or epoxy. Is one type of glue better for filling
stone flaws? Should I fill them before I work on them. I
only have old 3rd hand carbide equipment so things will be

Thanks for any help.
Jerry Kinder (new to the list)

Subject: NEW: Star Garnets

It was said in #224 about star sapphires:

<<The crystals are oriented along the molecular axes of the
sapphire crystal, which has six-way symmetry. Light is
reflected by the shiny rutile crystals into the six-pointed

Now, how about star garnets? Why are some 4 ray and others
6 ray? Or are they all 6 ray with rays too weak to see in
one direction?


Subject: RE: Rainbow Obsidian Structure and Colors

[I sent my query on Rainbow Obsidian to Bill Cordua, and
asked him to help. The following is his reply.]


Thanks for the note on the obsidian. I know less about
rainbow obsidian than I thought, and have found little
written on it in standard textbooks.

Obsidian, or any volcanic glass, is really a supercooled
liquid. Most of it has no orderly internal (crystalline)
atomic structure, such as minerals show. The common
interpretation for obsidian is that it is rapidly cooled
magma, so rapidly cooled that crystals don't have a chance
to form. Undoubtedly lots of obsidian forms in this fashion.
However, there are very thick lava flows, such as those in
Oregon or Yellowstone Park composed of obsidian. Given the
rate at which magma cools, there really isn't a good way to
explain how such thick obsidian flows would cool instantly
upon contact with the air. Obsidian is a very dry magma
(virtually no water) compared to other volcanic rocks.
Perhaps the unusually low water content discourages crystal
formation and allows volcanic glass to form at somewhat
slower cooling rates than is often assumed.

Now - to your rainbow obsidian. Volcanic glass is not really
very stable chemically over long spans of geological time.
It tends to spontaneously form small crystals, loosing its
luster. This "devitrifed" glass becomes more like the gray
matte surface you describe. Devitrification is encouraged by
water, so glass will devitrify more readily next to
fractures that allow ground water to penetrate into the
glass. This accounts for the top and bottom texture of your

Although I said above that obsidian formed too fast for the
formation of crystals, nothing in nature is so cut and dried.
The magma was bearing tiny crystals in it that formed on the
way to the surface. It is the light interaction with these
inclusions in the glass that lead to obsidian's optical
effects. I have a excellent little reference book entitled
"Rocks and Rock Minerals" by R.V. Dietrich and B.J. Skinner
that states: "The colors [of obsidian] are dependent upon
uniformly dispersed submicroscopic particles of magnetite
(black), hematite (red-brown) or tiny bubble holes (commonly
giving a gold colored sheen). " It is the distribution of
such substances that give rainbow obsidian its colors.

The striations you mention are a bit harder for me to
explain. Your proposal that these relate to layers within
the obsidian is probably correct. These layers are called
"flow bands" and are produced as the semi-solid glass is
sheared by movement of the still liquid underlying magma.
You know how glass gets when it's warm. This can produce
differences in composition, bubble distribution and
fracturing, which would then be emphasized by even minor
weathering. These differences would also help explain why
cutting the obsidian with regard to the layering would
produce the different color effects.

So there is my 2 cents worth on a beautiful summer day. If
you want this or an edited version for the digest, feel

Best wishes -

Bill C.
Dr. William S. Cordua
Professor of Geology/Mineralogy
315 Ag Science
University of Wisconsin - River Falls
River Falls, WI 54022
"Speak to the Earth and it shall teach thee" - Job 12:8

Subject: RE: Rainbow Obsidian Structure and Colors

The grooves in the obsidian are due to differential
weathering of the rock. The rainbow effect, if I remember
right, is caused by little mineral crystals lined up along
a flow line. (the groove) If you cut absolutely parallel to
the flow structure you will get a cabochon that will show a
bulls eye effect. If you would rather have bands of color
across your stone, cut it at a twenty degree angle. You
can probably see some sheen in the conchoidal fractures
where they are at that angle. Try shining a flashlight on
it from various angles.

Rose Alene McArthur

Subject: RE: Rainbow Obsidian Structure and Colors

Hello Hale,

Hope I can help more, but here's some info:

Obsidian is volcanic glass. A once molten, natural,
amorphous structured material.

The variety Rainbow Obsidian exhibits a phenomena, as you
described, iridescence (light interference effect in thin
films of gas or liquid, causing the "rainbow effect" on
the stone).

Obsidian's identifying characteristics are: gas bubbles,
crystallites, stubby needle-like inclusions, some exhibit
banding (as you described) or have numerous oriented
inclusions that cause a sheen.

Cause of color for Moldavite is iron, but unfortunately, I
don't really know of just one definite component for
Obsidian since it's so variable.

Rhodora de Jesus-Maiolo, G.G.

Subject: RE: Rainbow Obsidian Structure and Colors

(Copied from Rockhounds Mail list, with permission.)


Sheen and Rainbow Obsidians: An Electron Microprobe Study
J. Greller and G.Ulmer, Northeastern Section, Geological
Society of America Abstracts 30(1):22 1998

This seems to provide definitive evidence for the cause of
the colors.

"Sheen" obsidian results from oriented, elongated rods of
glass (not bubbles) of differing composition than the bulk
glass of the obsidian. Difference in refractive index causes
the effect.

"Rainbow" obsidian results from elongated, oriented rods of
augitic pyroxene. The large (up to 0.27) difference in
refractive index between the augite and glass causes the
"rainbow" effect. Augite rods are rarer than glass rods and
this is reflected (pardon the pun) in the rarity of "rainbow"

Henry Barwood

Subject: RE: Rainbow Obsidian Structure and Colors

Hale, In Oregon and Northern California there is lots of
Rainbow Obsidian. It all seems to come in banded versions,
some bands thicker than others (or multiple bands real close
together to form a thick band). These bands have the colors
in them and by polishing parallel with the bands you will
get a rainbow effect on the face of the cab. That is assuming
that one of the bands is on top or near to the top to show
color through the topmost layer. Thick bands are the best
for nice color.

Have fun.


Subject: RE: Rainbow Obsidian Structure and Colors

I cut some of the rainbow obsidian which came from glass
buttes in Oregon at 15 degrees off the layer lines and it
came out with a good rainbow. It works just like soft glass
so be careful of pits when you polish.


Subject: RE: Parts for 6" Sears and Roebuck Gem Maker

Hale, I've got a Sears Companion 6" Lapidary Outfit Model
#861.1405 and can furnish a pic of it if you think that it
would be useful for comparison purposes.

That is, I can furnish a pic when I get my Sony Mavica back
- I loaned it to a friend going on vacation to take pics of
friends and family...


Subject: RE: Does Alunite Need to be Stabilized?


The Alunite that I have worked is white to yellow/white
with deep red slashes. It polishes up beautifully and
while I wouldn't wear it in an abusive atmosphere, I
wouldn't hesitate to wear it in a belt buckle, but not in
a ring. Even tho the stone is soft and works easily, it
still is hard enough for pendants, etc..

Also, the stones I have polished aren't porous enough to
use stabilizer. All that I have was bought at Rock Shows
and seems to come in all sorts of shapes and sizes vs
layers or slabs. The stone holds a nice polish!

Just one man's opinion!


Subject: RE: Does Alunite Need to be Stabilized?

Hi again,

I didn't mention, in my prior letter about alunite, that
there is a fair sized mole hill of it just out of
Quartzsite where it was available to dig. I believe it is
now under claim. We dug a lot of it many years ago. (We
have a couple of boys who are back hoes.) Bob made a lot
of cabs of it and we all really liked to looks of it. But
it never sells well. There is something about the lack of
color I think that makes it unappealing to some people.
It does work nice for scrimshaw and carving because its so

I did some scrimshaws on cabs after they were polished and
they turned out well.

Micki Bleily
I said earlier that I had never seen Alunite; I was wrong.
At the EFMLS show this past weekend, I saw some for sale
which was brick red and white, and realized that I had seen
it lots of times before. I had passed it by as I had not
thought of it as an interesting stone. Now I see from your
note that others feel the same way. There may be rough
from other localities which have a difference appearance,
but what I saw didn't appear to me to have "character".

Subject: BIO: Alicia Carr


As you know I am new to the list. My name is Alicia Carr.
I am 18 and I live in Northern Idaho. I have just discovered
an outlet for my passion of rocks, lapidary! Basically I am
a very new beginner but I am very eager to learn. Opals are
my dearest love, followed closely by any type of feldspar
with fire glowing inside, but really I would love to work
with just about anything. I am looking for any tips, advice
or any friendly correspondence on the trade .

I would even appreciate stories of how any of you got
started. I am looking forward to any type of tidbit that
will get me more acquainted with this intriguing craft. I
am also very curious as to the tools or equipment I should
invest in for a very novice beginner. I hope to soon become
apprenticed to a very learned man who has his own rock shop,
we shall see.

Thank you



Subject: WTB: "Yellow Cat" (Redwood) Petrified Wood

My husband collects petrified wood and he asked me to ask
if anyone has any of this "Yellow Cat" (Redwood) rough
petrified wood for sale--it is found in the Henry Mountains
of Southeastern Utah. He may be interested in polished
pieces. Please respond off the Lapidary Digest.

Thank you.

Vi Jones

Subject: FS: Obsidian Chunk

Would anyone be interested in buying a five pound piece of
obsidian? You can make an offer if you are interested.

joshua wilson
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