What stones are safe to cast in place with silver / gold?
Generally you should look at the coefficients of expansion of the materials used. These are easy for pure metals but can be difficult to find for some alloys and gemstones. If the expansion/contraction information is similar then the chances of cracking are minimised. However, be aware that tiny imperfections in gems such as inclusions can greatly increase fractures. Also heating can irreversibly alter the color of some gemstones eg amethyst and zircon. Only experiment and the type of equipment used can inform if success can be achieved. I would suggest trying modern pewter (about 97% tin) m.p.223 C or even lead (higher temperature still) in experiments. But don’t contaminate silver or gold with the latter two metals.Good luck!