Material adhering to gemstones

Over the years I have noticed when cleaning customers jewelry that on
occasion I will get a sapphire or emerald that has collected a film
on its surface that defies normal cleaning practices. The film has a
look similar to an oil slick on water. Tripoli polishing compound
will remove it and it does no apparent harm to the gemstone. What
ever the material is that the client is coming in contact with is
very tenacious and seems to effect only specific gems. I have not
been able to find a commonality among my customers that would
indicate what would cause it. Has anyone else experienced this or
would know the cause? Any help would be appreciated.


It’s an oxidation surface that is created through long or repeated
contact with an acidic environment…like perspiration. As you
note, tripoli or diamond paste will remove it.


many emeralds are treated with waxes, oils and/or resins to
strengthen, improve colour depth, and stabilize them- the idea is to
reshine them, not to remove that film…it’s protecting the soft
emerald from steam explosions,oven temp changes ( thermal shock) and
other chemical and organic compounds that wreak havoc on the brittle
soft nature they possess…sapphires are generally just dirty,and can
withstand a different type of cleaning than emeralds…


I will get a sapphire or emerald that has collected a film on its
surface that defies normal cleaning practices 

Jefffrey - There is a phenomenon called variously, but usually boric
acid etching, but it can’t be on emeralds. When someone retips or
solders on or near corundum, and uses a boric acid dip like usual,
the boric acid can etch the surface of the stone under heat. The
more heat, the more likely it will occur. The effect is just as you
describe - often it looks like motor oil on the surface of water,
the pattern of it. Sometimes if it’s light tripoli will dress it up,
but only refinishing the surface will truly remove it. I suspect
that you are seeing that at times, and other things at other times.
It can’t be on emerald because you can’t heat them and it only
happens on corundum. It’s pretty common though, because less
experienced repair people don’t know, and then they think it’s just
tenacious dirt or don’t see it at all. If you’re heating corundum
just don’t put boric acid or wipe it off the stone before heating it

  • corundum doesn’t need the coating anyway.

Hi Jeffrey,

As a website owner,, and Graduate Gemologist (GIA),
I receive this question very often. What you are seeing is actually
just what you described, an oily residue that has built up on the
gemstone. Gemstones tend to attract oil and grease from the skin of
the person wearing it, as well as from hand lotions, certain soaps,
etc… The best way to clean these gemstones is to clean them with
denatured alcohol (not rubbing alcohol) and wipe them dry with a
clean Bounty paper towel, as it is lint free. This is a very cheap,
safe and effective way to clean the oily residue build up on
I hope this helps.

Linda McMurray G.G., A.J.P. (GIA)