AFM says they are made from borosilicate.
You can also use the Elev8 diffuser beads as a terp pearl. They are a little larger but work great, they hold more heat so in theory, they are better than a smaller one. The Elev8 diffuser bead is made from soft glass. We have put a torch to them and they did not break. We feel for the price, they are amazing.
Learn even more at our diffuser bead blog
See them as a terp pearl