I’ve used garnet sand several times to hold pieces in place while
soldering and it definitely acts as a heat sink, although that was not
my goal at the time. I imagine it would work as well for protecting
stones as several of the other heat retardant methods mentioned here
For those who are unfamiliar with garnet sand, it is exactly what it
sounds like: small grains of crushed garnet. Once you mix it with
water to a thick paste consistency, you can place parts to be soldered
into it, being careful not to get any right on the areas you want to
solder. It dries/hardens quickly and will hold your parts quite
firmly. It’s certainly faster than using investment, though not as
rigid when dry.
Another nice thing about it is that you can crumble it up when you’re
done and reuse it.