Reusing an 8-sided die in a rolling cage to function as a 6-sided die. See the video on youtube below for a short explanation on how it works.
Only available in steel. Most materials are too brittle for this structure and will break easily. White, strong & flexible is strong enough, but the pips are hardly visible and it rolls less well.
The steel also explains the price, since it's about 2 cubic cm in volume.