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Pitted onyx


#1

One of my clients came to me with some old favorite polished
"onyx" beads she’s worn for many years, and some of the beads’
surfaces are rough and pitted–the ones that would make contact
with skin of the client’s neck when worn. (The ones that would
fall on the chest are not rough or pitted, and retain their
polished surface).

I’m trying to figure out what caused this. Haven’t this in a
harder stone before. Would the spray carrier in perfumes/colognes
do this to an agate? Some of you more experienced stone folk have
a thought?

If she wants to restore the finish, should I get someone to
tumble them? (They’re only 8mm). In what medium?

Thanks for any thoughts.
Colene


#2

Have you tested these beads? Are you sure they are all onyx?


#3

Could be that it really is onyx, rather than dyed agate. Onyx is
a form of calcite. One can drop it in acid and watch it fizz. If
your client’s perspiration is acid, it could cause the pitting
which you observed.

Lee Einer


#4

Oh No. True onyx is a black and white banded agate, SiO2.
Because of the increasing scarcity of this material, just the
black portion is used and called onyx. it is not etched by acids.
There is a variety of calcite called travertine that has bands
and sometimes resembles true onyx. It is properly named
"travertine onyx". It is CaCO3 and is etched by acids. Will
Estavillo, www.natureshop-gallery.com


#5
Looked up onyx composition. True onyx is calcite (calcium
carbonate) so would slowly dissolve in perspiration if the skin
was at all acidic. FWIW Geo.

Oh No. True onyx is a black and white banded agate, SiO2.
Because of the increasing scarcity of this material, just the
black portion is used and called onyx. it is not etched by acids.
There is a variety of calcite called travertine that has bands
and sometimes resembles true onyx. It is properly named
"travertine onyx". It is CaCO3 and is etched by acids. Will
Estavillo, www.natureshop-gallery.com


#6

Thanks, Mr. Spirer and G. Stanford, for your response to my
query. Also for the heads-up on the beads being (some of them
at least) something other than onyx… I did a discreet scratch
test, and many of the beads (the unpitted ones, in fact) are
above 7; the pitted ones are easier to scratch with a knife, but
this might be a function of the surface being pitted–in any
case, none tested below 5-6. Where one or two of the beads has
been chipped, the fracture looks glassy, (almost like glass or
obsidian, though they are truly opaque with a polished surface).

G Stanford just posted that “True onyx is calcite (calcium
carbonate)” and that this could indeed pit from perspiration. I
could buy this, if I thought these were calcite. I just did not
consider that they might be, because of the hardness–calcite is
3, and they test between 6 and 7. “True onyx,” could be
something other than what this seems to be, the black stone
popularly called “onyx,” which is an agate (possibly dyed) with
a hardness of 7-ish. I (perhaps mistakenly) assumed a
mostly-quartz stone would be relatively unaffected by acids and
salts in the skin. Maybe I need to go back to the books,
myself!

These beads are pretty old–should we expect that perhaps even
relatively hard stones can be eventually eroded by salts and
acids in perspiration, perfume, etc–given long years of wear?

I’m learning, and I appreciate the help. Colene

Colene Abramson
Repair the Old * Create the New * Jewelry and Leather
508 358-3538 Fax 508 358-7654 @colene


#7

Oh No. True onyx is a black and white banded agate, SiO2. I stand
corrrected. I guess that one can’t believe everything they see
on the internet. Geo.