Agnes, when it comes to the mineral hardness of coprolite, I think
you’re at the mercy of dino species (equals a varied dino diet), what
soil it landed in, subsequent metamorphosis, what mineral replaced
what, etc. Coprolite is not a resin, as a previous reply suggested.
I have found that, when cabbing most materials, one should expect a
change in hardness (and thus undercutting, etc) whenever there is a
Ask me about my favorite piece of Munjina from Australia! I worked
for hours to polish that little jasper! (Yes, a pun. A bad pun.)
Those lovely color changes (think Crown of Silver psilomelane) are
often the areas that the grinding wheels will catch, drag, and
splinter. So when I have doubts about a much-loved but rascally,
unfaithful stone, I start on the finer-grit wheels and work there –
even for the initial shaping. Of course this can add hours to the
work time. Fortunately I do stones chiefly for love.
Another point to remember is that you’re not just grinding with
whatever grit your wheel is charged with. Unless you have a copious
water supply continually flushing your grinding surface (such as the
CabKing cabber uses) you’re also grinding the stone with whatever
grit is in the swarf from the part of the stone you ground 10 or 5 or
1 second ago. So if you use a flat lap, copious water plus stopping
for a periodic flushing is good.
You can also ‘stabilize’ the material first with any number of
glue-like products (as with flowering tube onyx), but of course you
should disclose that to whomever buys what you sell. Sometimes it’s
necessary to stop work and stabilize again (think plume agates) and
maybe even a 3rd time, until final polish.