Metal clay is a squishy substance made of water, a binder, and particles of metal – the one I use is specks of silver. You can mold it or cast or layer it or sculpt with it, it's clay. When you're done forming the piece, you give it time (and maybe a little bit of heat) to get rid of the water, then you fire it to burn off the binder, leaving only pure metal behind.
My favorite thing to do is sculpt little creatures in polymer clay and make a silicone mold of them for casting the silver.
I can add some kinds of gems by embedding them in the clay, but others are not suited to the high temperatures that firing off the binder requires.
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