While I always do scratch and SG tesdts to separate out like
looking stones, heft is a good first indicator what you have.
Don, I surely hope you're not doing scratch tests on cut stones, are
you? Destructive tests like that, while OK for rough, are usually
considered a bad idea on cut stones. Istead, might I suggest a
refractometer? Though they're a little pricey, they're easy to use,
and a highly useful test. But just a polariscope (which you can
build yourself if you like) will give you many of the seperations when
you combine it with SG, which you're already doing. If you check SG
with heavy liquids, instead of the harder to use scales, you can
relatively easily make up liquids calibrated for specific
seperations, such as aqua from topaz, etc. Very fast to do, and no
damage to the stones that way.