From the Manufacturer,
6mm Ruby Spheres.
"ruby sphere by cryptic culture. add nucleation action , surface area, and heat retention. we are offering two sizes 5mm for smaller quartz, and 6mm for larger.
these are lab/instrument grade rubies. although these are very robust and have an excellent high temp working range, do not toss glowing rubies into an alcohol bath.
ideal for your directional spinner caps."