What magnification do I need to read an inscription on a gemstone?
It should be possible to calculate this, but note that it will depend on your eyesight as well, and the contrast you can get (directional lighting may help) 

If you look at how many characters there are in a typical inscription (I've seen up to about 30 for synthetic diamonds online;this is lovely to be a worst case), and how much space they fit in, you can work out the character spacing. 30 characters in one row of 5 mm would be 0.17 mm character width including separation. 30x would make this equivalent to about 5 mm which is very large print. Normal text is more like 1-2 mm per character, which you'd achieve with a 10x jeweller's loupe. If you're collecting gemstones you probably want a good loupe anyway, and if this doesn't allow you to read the inscription, it should at least allow you to count the characters and rerun my calculation.

I do suggest getting a decent loupe. Mine is only a cheap thing, but at 10x still allows me to resolve features down to that sort of scale with decent eyesight; reading inscriptions of about twice the size I calculated above (on electronic parts) is trivial. I don't have a gemstone, let alone an engraved one, to test for certain.

You could also resolve this with a decent camera with a macro lens or extension tubes. If you have access to an SLR a cheap set of extension tubes would allow you to record the inscription. Holding everything steady and focussing can be more of a challenge