To produce an homogeneous dark glass I melted sediments in an electric furnace at a maximum temperature 1400 °C for about 4 hours. Then they were casted in a steel plate and annealed at 550 °C. I don't know about glass pellets, but I can provide some link with industries if you contact me. good luck !
To do our ICP analysis we're fusing our rock samples in platinum crucible. The rocks is melted in a fluxiclaisse appartus. if you would like to keep a glass bead you just let it cool. We rather pour the molten product in a mix of acids and water in a Teflon vessel.
Platinum-Rd and Alumina crucible can withstand high temperature (up to 1400s C). Alumina crucible is cheaper than platinum but you might get some Al leaching into your melted silica.