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This list digest contains the following message subjects:

1. LapDigest News for Issue No. 43 Sun 8/10/97
2. NEW: Polishing Opals
3. NEW: Drilling Fragile Stones
4. RE: Polishing Rainbow Obsidian
5. WTB: Petrified Wood Table Tops
6. WTB: Petoskey Stones
7. AD: Sapphire Faceter for Sale or Trade


Subject: LapDigest News for Issue No. 43 Sun 8/10/97

The ruler above 60 spaces, and all scrolling problems seem
to be licked if the Digest fits within that space. I think
Dave's printing problem remains. After looking at this
issue, if any of you have scrolling or printing problems,
please let me know what they are, the computer you use,
and the software you use to view the Digest, and we will
continue to work on those problems.

Please remember to send me any questions about physical
properties, such as hardness; we will do the 'Measuring
Hardness' issue this week.

And please send me any Vibrating Lap questions. Send these
and other questions to the address shown on the masthead.

Finally, I will be away for a week during which time the
Digest will not be published, starting Monday August 17th.
Will be taking Opal Cutting up in the Blue Ridge Mountains!



Subject: NEW: Polishing Opals

I just got a large amount of Mexican jelly opal and the
wooden spindles to polish them. I have never used them
before. Any ideas on how to start? I have used diamond
wheels and silicone carbide belts to polish jasper and other
material before but this is new to me.

Mark Case
Woodmen Summer Camp
Randleman, NC

Subject: NEW: Drilling Fragile Stones


I have a friend visiting on a South American buying trip
from the Czech Republic, who is a dealer in rare gems
(here for clear cassiterites). He is having trouble drilling
small and fragile pieces by conventional means & asked me to
post a request for information about ultrasonic drills (or
any other suggestions). Cost is important- I suggested he
check around Idar Oberstein and he said "too expensive,
please ask some Americans".

Therefore....any recommendations would be appreciated. Many

Cochabamba, Bolivia

Subject: RE: Polishing Rainbow Obsidian

In Issue #42, Ken Wetz ( answered a query
about polishing obsidian. He said, in part: "The 80 grit
might be part of the problem, it will leave stress fractures
that extend into the stone and you might not grind far
enough to remove them. Even a 260 <grit> will leave some
minor surface fracturing. Remember, what you are doing is
gouging out scoops of the rough as the diamonds make
contact and this will cause cracks and fracture lines that
extend into the stone. Also obsidian can have lots of small
inclusions, both air cavities, and trash that might be due
to partial crystallization or something that the flow picked

I noted that: "Ken's explanation sounds so reasonable for
obsidian that it makes me wonder what other materials
behave this way, and what are the properties of the materials
which make them behave this way - conversely, why don't all
materials behave this way? -- or, do they?" I asked Dr. Bill
Cordua, a geologist at the University of Wisconsin at River
Falls, about this, and he gave the following answer:

Bill said: "Obsidian, being glass, exhibits conchoidal
fractures which accounts for the scooped out shapes. Lots of
other minerals have this sort of fracture - quartz is a good
example. I'm thinking obsidian will be more vulnerable to
this because it is a glass not a crystalline material like
quartz. Under the microscope, obsidian is full of small
curving fractures called perlitic fractures. These are little
cooling cracks formed by contraction. The obsidian eventually
will crystallize, usually forming a dull gray mix of
intergrown quartz and feldspars. The perlitic cracks aid in
this process by allowing water to penetrate into the
obsidian. The first of the crystals to form will grow along
these cracks. They will be microscopic, but will encourage
the obsidian to break along these surfaces. That could
account for the spalling. Quartz and other minerals don't
undergo this "devitrification", so wouldn't be so
vulnerable. BTW, "Apache Tears" are obsidian fragments whose
outer shapes formed along larger perlitic cracks.

Your friend is also right about the presence of
diverse inclusions in obsidian. Most obsidians were beginning
to crystallize before they stopped moving as lava, and have
through them. Quartz and other minerals have inclusions too,
so I don't think the inclusions themselves have a lot to do
with the stated problems."

Dr. William S. Cordua
Professor of Geology/Mineralogy
University of Wisconsin - River Falls
River Falls, WI 54022
"Speak to the Earth and it shall teach thee" - Job


Subject: WTB: Petrified Wood Table Tops

Hi All,

Do any of you cut and polish table tops from cross sections
(20 - 40 inches) of petrified wood ?. I do not (petrified
wood is quite scarce in Denmark), but I would be interested
in acquiring such a piece.

Greetings to y'all

Jan J. Hansen
Copenhagen, Denmark
..sliding down the razor blade of life (Tom Lehrer, 1959)



Subject: WTB: Petoskey Stones

I have a client interested in acquiring 100-200 pounds of
Petoskey Stone from Michigan. Stones must be of a large
size as he wishes to sphere them. If anyone is interested
or can advise a source let me know and I will put you in
touch with the client direct as this is a favor.
Thank You and best wishes.


Subject: AD: Sapphire Faceter for Sale or Trade

As part of an estate sale, I got an older Sapphire Faceter
today. It needs new laps and a drive belt, but the rest is
working well. Is anyone interested in purchasing a Sapphire
or trading? Please email me privately.

Mark Case
Woodmen Summer Camp
Randleman, NC
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