4-6mm diameter Lauscha cryolite white opaque rods. (Kryolith in German).
Working tips: avoid using this with Lauscha's dark transparents (such as black - they list it as an opaque but all blacks are really transparents). Light transparents and opaques should be fine.
In my test, cryolite white dots on SNO 480 black seemed ok, but the other way round results in compatibility cracks.
Average weight per rod: 18g