Interesting question. I’m not a geologist but it’d be fun to think
this one through.
What makes a stone good for knapping? Well, it needs to be hard,
which sapphire is, it needs to be pretty tough, which sapphire is,
and it needs to evince conchoidal fractures. That’s the rub.
The traditional materials for knapping are flint, of course, chert
and obsidian. Flint and chert are microcrystalline rocks, and
obsidian is amorphous. So the question becomes, can a mineral of a
single crystal structure, such as a large sapphire crystal, be broken
in such a fashion that useful cutting edges are achieved.
My guess is no. I think it’d just split along its cleavage planes
and you’d not be able to get a good edge out of it.
However, any of the cryptocrystalline quartzes should be good for
flint knapping. Banded agate, chalcedony, onyx, they’d all make very
pretty knives and arrowheads. Of course, they’re pretty tough so the
pressures needed might be very different from flint or chert.