I’ve seen a few. To start with, I’ll say the things are harder
to spot than C.Z. The C axis of the material is generally
perpendicular to the table, so you don’t see facet doubling
straight on. but it’s clearly visible at a 45 degree angle to
the table, looking into the crown at back facets. And while it
doesn’t display that dramatic "obviously something wrong"
dispersion and dark look of C.Z., a trained eye will still find
that it’s not quite right. Reflections, dark areas, etc, aren’t
quite where you expect, though you might have a hard time putting
your finger on just what or where. The hardness is good enough
so that the obvious rounding on facet edges and the girdle is not
there, but under a microscope, you can still find enough rounding
of facet edges to at the least, make you suspicious. The facet
doubling is the big clue. Table down, the appearance also is
wrong, but this is something you have to learn to see.
Note that the facet doubling can be difficult to see with a
loupe, in smaller stones, and also in some non-round stones. I
almost missed it in a carat sized princess cut, even when looking
for it. Had I not already known it was a moissanite, I might not
have looked hard enough to see it, with just a loupe. Under a
microscope it was obvious, but just sitting at the bench, not so
much. And a small (fifteen point) side stone was small enough
that even careful looking with a loupe left me unable to see for
sure whether it was a diamond or not, even knowing it was not.
Again, under the scope, it was obvious.
With loose stones, a bottle of methylene iodide is useful.
Diamonds sink in MI. Moissanites float. (C.Z. also sinks, much
faster than a diamond) Be reasonably careful with MI, however.
It’s a somewhat toxic liquid. GIA/Gem Instruments sells small
bottles of Methylene Iodide intended for such specific gravity
testing if you need a source.