I have only been partially following this thread so please forgive
me if this has been mentioned already.
If you prefer not to use any potentially dangerous or irritating
chemicals, you can always "frost" stones with controlled
air-abrasion (sand-blasting). Use different medias according to the
hardness of the material being etched i.e. glass beads for soft
stones, aluminium oxide for medium hard stones, and silicon carbide
or boron carbide or diamond for very hard stones. With
experimentation and a little practice it is easy to achieve the
desired results. In my application, I use #180 grit aluminim oxide
for preparation of surfaces before adhesion. A smaller particle size
would give a gentler, shallower, etching which may be more
appropriate for your needs.
Danville Engineering makes an inexpensive, under $100,
micro-sandblaster. Their tel. is (800) 827-7940, I have no relation
here other than I use their product. You will still need a source of
compressed gas such as a compressor or CO2 bottle.
To protect areas where etching is not desired, I have found that
modeling clay does a great job and is more effective than tape in
Steve Green - Rough and Ready Gems
your source for fine briolettes www.briolettes.com