I am teaching a class today and one the of the most common questions is what stones can I fire in metal clay? This is a great question and the answer can be quite confusing to a beginner. First of all remember that when you fire silver metal clay you will do doing so at 1200F to 1600F. Many materials cannot tolerate this high temperature. There are no clear cut rules on what stones can or can't be fired in the clay. Luckily there are people who have experiemented with a variety of stones in metal clay and come up with a list that you can refer to. My facorite reference is the following from CoolTool.us titled Gemstones in Metal Clay. This reference includes a list of temperatures and methods of firing a variety of stones.