A couple of years ago a box with a tortoise shell veneer was shown on
"Antiques Road Show". Much of the surface of the veneer was dry and
dull looking. The antique expert explained that the problem was very
easy to fix, and she demonstrated by gently rubbing in a small amount
of olive oil.
It worked on the TV program and I promptly tried in on a very long
necklace of tortoise shell beads that restringers had “washed”,
removing all the shine and leaving a very dull surface that
eliminated the beautiful play of colors the beads had originally
had. (I suspect they “cleaned” the beads with an ammonia-detergent
mix of some kind.)
I tried the olive oil trick, and the polished look and the glowing
colors came back almost entirely after two treatments. (Olive oil
on a clean cloth rubbed on the beads.)