Difficulty with drilling holes in Florida coral

I am having difficulty drilling a 2 mm hole in some Florida coral.
The pieces of coral are about finger size. They are solid agate. The
pieces are about 1/2 inch thick. I am drilling them like I drill my
regular agate material but I am wearing out my drill bits. What is
different about the agate in these pieces of coral.

I wanted to make some bead from these pieces of coral. I was going
to tumble them after the whole was drilled.

Larry E. Whittington
Larry E. Whittington Lapidary

Larry there are differences in fossilized materials as you have
noticed. The coral itself is usually replaced with a different
material than the surrounding elements. So everything that was soft
in the coral such as the polyp is now replaced with agate, but the
silica based exoskeleton is replaced with a different material. I
don’t remember enough of my paleontology without the books to tell
you what the likely chemical composition of the replacement material
of the hard body is but if you should be able to google it.

Good luck. By the way the body of the coral should be replaced with
a hardermaterial than the agate or the agate replacement would
destroy too much detail in the coral. If I remember correctly.


I’m confused. Coral, at least most of the coral we jewelers work
with, is calcium carbonate, while agate is quartz which is silica
(silicon dioxide).

Mike DeBurgh, GJG
Henderson, NV

Hi Larry,

When I drill into agate, and most any stones, I use a diamond bur.
They come in many shapes & sizes, but I like my very tiny flat top
bur (as apposed to a pointed diamond bur tip). It is very small,
less than.5mm. I purchased this through jewelry artist Michael Boyd
at a workshop, but I know he gets them from diamond Pacific. They
come in various grits as well. They are great and can fit into a
small drill press or into the flex shaft. They are also reasonable

I know you can also get diamond imbedded drill bits for glass and
stone, but I find that no matter what size hole I will eventually
need, that if I start with this little bur I don’t get cracking and
unsightly chipping of the hole’s edge. I also will use the
appropriately sized ball bur to camphor the edge before I move up to
the size of the actual hole I will want. This is for the same
reason, no chipped edges.

They are called “Dumont sintered carving points” and are excellent.
You can get a pdf file of their 2010 catalog heRe:


The drilling point I use to start & drill the initial hole is part
number 103-0734 on page 83.

Have fun and Good Luck!

Hi Mike,

I am sorry I confused you about your coral and what it is made from.
I am new to learning about stones and their make-up and not at all an
"expert" at naming the compounds that make up most stones. I was
simply meaning to inform you about the use of diamond imbedded burrs
and how they can cut through agate very easily, and how they would
also be suitable for drilling into your coral. I was just trying to
give you a suggestion for what to try to drill your coral with.

Also, depending on where some corals are found, say in the areas of
dry desert or mountainous places like Colorado, maybe someone could
tell me. can they “grow” or form similar to agates? I have seen some
corals and other fossils that appear to have areas in them that are
very hard and look like silica. I am still a novice when it comes to
knowing the base material of many stones, so this is interesting to
me as well.

If any of the stone experts here want to comment on what exactly the
make-up those types of coral is, feel free, please, as I too am
learning and I soak up the I find here like a sponge.


Hi Teresa,

When someone speaks of “coral” I believe most gemologists would
consider it to be primarily calcium carbonate and something harvested
from the sea, black and golden corals excepted; they are not
calcareous, but made of a horny type of material. If it is a coral in
which silica predominates then a qualifying descriptor is usually
added so the distinction is clear, such as Agatized Coral, Fossilized
Coral, etc.

All coral is produced by animal communities in the sea. You can read
about them being threatened in the news regularly. In a manner
similar to wood, once underground, in the right conditions, and over
many years, the original material might be replaced by silica.
Petrified wood is an agate variety of quartz as is fossilized coral
which has had its original material replaced by silica in a similar

Mike DeBurgh, GJG
Henderson, NV


Can you tell me if black “tree” coral has any value for use in
jewelry. It is pretty if I polish off the roughness from the skin?


If the black tree coral you speak of is the same as what we call
black whip coral here in sunny West Australia, I can’t say for sure
what dollar value it has in jewellery - but it does make up into
beautiful pieces, whether you remove the little spines or not. I was
given some that came up in a prawn trawlers’ net, and, once all the
sea detritus was cleaned off it, the long, thin, spiny bits had a
beautiful high shine without further assistance. I had no difficulty
drilling mine, as long as I drilled slowly - otherwise, it had a
tendency to begin smouldering, just like wood.

As others have said before on this forum, coral of all kinds is
under terrible threat today, and its continued existence and health
is more important to planet earth than most of us realise. I would
not even have accepted the piece I was given if I had not known and
trusted the person who gave it to me.

Check out a couple of pieces on my website -

Jane Walker

Hi Georgia,

There are ways you can interpret “value”, because in my opinion,
with art jewelry the value comes from the work itself, the design,
the Artist’s creative vision, etc. etc. all these things add value.

I go to the SNAG (Society of North American Goldsmiths) convention
every year. There is a gem cutter there by the name of Bill Gangi who
sells cut and polished black coral pieces and branches (and red) by
the carat weight. It is truly beautiful. And yes it does polish up
very nicely. When used in Art Jewelry I feel it adds to the value. He
sells quite a bit of it, too, so there are many other jewelry artists
that would perhaps consider black coral to be a semi-precious stone.
Perhaps the value is in the amount of time he spends to cut, shape
and polish it.

As far as the dollar value, I have to admit I have absolutely no
clue whether this is really valuable. I am a novice gem cutter at
best, and just learning the ins & outs of gems, their make-up, and
their market “value”. It all seems to me to be dependant on your
viewpoint. Take diamonds for instance; They are not nearly as "rare"
as the market tells us they are. Not to nudge anyone’s sensitivities
here, but to me it seems that diamond values were and are very much
created by the market itself, and of course then there is the whole
issue of how they are obtained, etc. (Hope I am not opening a can of
worms here! I am not intending to spark a heated debate, which I see
often happens here in Orchid-Land)

I think value is very dependant on your point of view. As for
whether Black Coral is marketable, I would say YES, as I have seen it
marketed before. What the exact cost per carat weight, I don’t
remember how much Gangi sold it for. I could look back at my records
to tell you what I paid for my red coral, but there might be others
here who can give you better advice on what to market it for if you
were to begin cutting and selling it. Sorry I could not give you a
better answer.


Thanks Mike! Ya, I kind of thought that the corals I have from
Colorado looked agatized. (?) proper term? Lol

It is very beautiful, and I do also have a large petrified wood
which I am currently slabbing up for cabs that came from near the
same area. I just love agates for some reason I guess, whether it be
from coral wood, or even fossils. They create such beautiful stones
when they are polished up. And with the little silica windows it
looks like you are looking into other worlds.

Anyways, I ramble. Thank You Mike, and all other Ganoksin-ers out
there! It’s like continuing education! I just love it and I am
addicted to Ganoksin!

Here in Las Vegas at the Venetian Hotel/Casino there is a store, the
Bernard J Passman Gallery, passman.com, that focuses on black coral
jewelry and sculptures. It can be quite valuable.

Mike DeBurgh, GJG
Henderson, NV