I'm actually trying to get together stock for a small online shop selling natural gems that can be fired safely in metal clay.
Diamonds are a no I'm afraid, but some gems can be kiln fired, some even fired by torch If you're careful, and they are eye clean (VVS_VS quality) like Garnets, Peridot, and white Sapphire's which are really pretty and sparkle almost as brightly as diamonds.
As for Rubies, I personally would avoid them because the quality of stone good enough for firing would be quite expensive. If you like red, Garnets are OK and Sunstone's can be found in Red and lighter pinks.
For kiln firing there's a bigger list including Tanzanite, Chrome Diopside and Sunstones, which are gorgeous, come in 4 or 5 different colours and can have beautiful copper inclusions that flash and sparkle.
Here are some recommended lists from the PMC guild and Art Clay World, but they are not 100% up to date:
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To be on the safe side I'd recommend kiln firing if possible though, even the best quality stone has a small risk with torch firing and can be ruined if you're not extremely careful and know what you're doing with a torch.