You can use a diamond bit in your flexshaft to enlarge the holes.
You don’t say what size the rubies are or how big you want the holes
to be, but you can get the diamond bits in a wide variety of sizes.
There are diamond drills, diamond-encrusted cylinders (what I’d
recommend for your purpose), and diamond core bits (great for larger
holes, in particular).
Keep the rubies and bit in water as you drill them and don’t keep
the drill moving too fast.
Hope this helps!
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