I have read that gemstones can be fired directly into the clay. Can I fire my diamond in it?
Most CZs can be fired into your projects. I have seen several failures with stones that were supposedly tested for the heat required for the metal clays. One beautiful diamond-like CZ became cloudy and had an opal appearance--very attractive and fortunately it worked with the design, but a failure nevertheless.
One CZ lost all of its facets and became a rounded piece of glass. This was a real failure, and I had no solution for fixing it.
In my studio, I test all stones personally before they are worked into projects. If a stone fires correctly the first time, it will fire the same way (at the same temperature) in each subsequent firing.
As for you diamond, the answer is no. However, you can place a setting into your piece and set your diamond in after the firing.