Back to Ganoksin | FAQ | Contact

Fossilized coral


#1

I was wondering if anyone knows how the fossilized coral I’ve been
seeing on eBay is cut and polished? Do they fill the voids with
resin of some sort before they polish it? I have lots of fossil
coral around here, and I’d like to make some pendants of my own, but
I don’t understand how they get such a good polish on the cabs and
beads I’ve seen listed.


#2

Depends on the particular sample of coral. Are you referring to the
Indonesian material (yellow hexagonaria-type)? If that is the
material, it is agatized and very showy, but I noticed you rarely see
rough for sale.

I cut a lot of hexagonaria/xystraphyllum (sp?) and favosites from
Prince of Wales Island, AK and have cut some from Florida. About
50% of it is cuttable, without cracks and will accept a good polish.
I don’t treat it, but I know a sphere maker who uses a sculptor’s wax
on it after polishing. It is a limestone, however, not agatized like
the Indonesian material.

Not sure if that is of any help, but…

Chris
Ketchikan, AK


#3

“Fossilized” coral is completely different from most regular
corals…

Many organic materials become fossilized, like a fine aged wine, in
this case thousands or millions of years, when they have had their
minerals of calcium, etc replaced with hard (polishable) silica…
In the case of bones and coral is essentially a bone-like structure,
or cellulose, in the case of fossilized trees, both have had silica
replace much of their original organic molecules…

Still, the coral you have may be polishable whether it is fossilized
or not… The same steps used to polish jasper or agates may be
used… If coral is fossilized, the silica content will determine
the polish gloss level and its continued ability to hold on to a
hard and high gloss finish… Soft stones will still polish, just
not as well, or for as long, depending on usage…

RocknLight


#4

Much of the fossilized coral is solid requiring no filler. I
recently saw some large slabs, around 1 square foot, that were at
least 80 percent solid with some porous areas around the edges. I
know some people use epoxy to fill voids in rock, I’m guessing you
may want to use a vacuum chamber in this process to ensure complete
filling. A product called opticon is made for filling fractures and
is very runny and may not require a vacuum, but I’m just guessing,
I’ve never used it.

HTH
Mark


#5

Re cutting fossilized coral…by now you must know there are many
kinds of fossilized coral. Some is nothing more than ‘old’ coral
that has hardened into a calcasious rock while other coral has
actually agatized and has become something else…like petrified
wood. If you have the former, some of it will polish but most of the
detail will be lost. It can be messy to work with.

The latter, the most famous coming from Tampa Bay, FL is a different
animal. Actually, petrified coral comes from many places…even from
Nebraska, Panama, Indonesia, Maryland, Tampa Bay and the
Withlacoochee area of FL - NE of Tallahassee, to mention a few.

I love the FL corals much of which is fully agatized. They can be
cut and polished as cabs, tumbled or slabbed. About 90% of it will
take beautiful polishes using normal lapidary procedures and
polishing with cerium oxide on leather or felt. These corals come in
many colors, bluish (mostly the Withlacoochee variety), red, orange,
yellow, brown, and jet black. Some are transparent and can even be
faceted if one is inclined. Size ranges from small branches to huge
heads more than a foot or more across. I love to cut the small
branches, which often are hollow, into rings and tumble them. Great
for all sorts of pendants, erings, etc. The Panama and Indonesian
corals (some very expensive…see New Era Gems selling the latter at
overt $100 a lb) and are simply beautiful. Huge flower like patterns
can be had in many of these corals and they make beautiful cabs or
even carvings.

Take a look around it is available still though the Tampa Bay
material is becoming very scarce. Our gem and mineral society had
about 400 lbs of mixed material but it has been pretty well picked
over by now. Still, Ebay still has some and go to Google and search
on coral. You will get a lot of extraneous (very
interesting) but can also find pet coral for sale.

Cheers from Don at The Charles Belle Studio in SOFL where simple
elegance IS fine jewelry! @coralnut2


#6

A geologist friend of mine, who also happens to be a beader, was by
when I read your post. She said that all coral is lime stone, which
is, of course “polishable” what makes this a fossil, she guessed
was that the actual bodies of the coral animals are also fossilized,
having been replaced by minerals over the millennia. They are,
however, microscopic. I did not ask her about the issue of color, I
know that a symbotic bacteria called zooxanthellae, which lives in
or near the coral bodies, is responsible for the color of coral.


#7
I was wondering if anyone knows how the fossilized coral I've been
seeing on eBay is cut and polished? Do they fill the voids with
resin of some sort before they polish it? I have lots of fossil
coral around here, and I'd like to make some pendants of my own,
but I don't understand how they get such a good polish on the cabs
and beads I've seen listed. 

I’m not sure about E-bay, but I think you need to find a better
grade of rough. If it’s porous, it’s not going to polish out well. If
you fill it with something, you certainly should disclose that to
buyers, although at E-bay “caveat emptor” seems to be the rule (don’t
get me started on the people there selling Colombian “amber” and
"unsearched" bags of Genuine Emeralds). There used to be good fossil
coral coming out of Michigan (Petosky stone), and you can still find
"Ram’s horn" coral that’s solid, with nice red colors. Most gem-grade
fossil coral these days seems to come from Indonesia; specimens I’ve
seen were in pastel shades: bluish, pink or beige, with good
definition of pattern and no voids. It wasn’t particularly expensive
either; slabs were running about $0.25/gm at the last rock and
mineral show I attended. Work it normally, using diamond until the
end, then cerium oxide wet on leather for a final polish.

Andrew Werby
www.unitedartworks.com


#8
She said that all coral is lime stone, which is, of course
"polishable" what makes this a fossil, she guessed was that the
actual bodies of the coral animals are also fossilized, having been
replaced by minerals over the millennia. 

Hmmm…I mulled this over with my husband, who used to teach
paleobiology.

It’s an interesting notion and not impossible, but most fossil coral
is devoid of fossilized coral animals (polyps). They’re very tiny,
soft-bodied creatures and would only fossilize under the rarest of
circumstances. Also, the coral skeleton is calcium carbonate, which
is the same material that constitutes limestone, but this doesn’t
necessarily mean that coral is limestone. The mussel shells in your
linguine alla frutti di mare are also calcium carbonate, but you
wouldn’t call 'em limestone until they had spent a long long time
being compressed into solid rock. Anyway, the material used for cabs
is this calcium carbonate skeleton, which has been augmented by or
replaced with other minerals over time.

Also, the color of living coral does result from zooxanthellae, but
the coral used for gems is just the non-living skeleton, which
probably gets its color from minerals or compounds (i.e., iron
oxide, etc.) present in its structure. It would be interesting to
know what, specifically, causes pink coral to be pink, red to be red,
etc.

What a cool thread!

Jessee Smith
www.silverspotstudio.com