These are faceted, lab-grown BGO20 (Bismuth Germanate), a rare crystal grown for opto-electrical purposes with incredible sparkle and fire.
This is a real cool rarity in the gem world: bismuth germanate (in this case, specifically Bi12GeO20 aka sillenite). BGO20 has bizarre opto-electrical properties which make it incredibly useful in niche applications, such as as an acousto-optical modulator. It also has the highest density of any gem you can get, with a specific gravity of 9.22 (ie 80% as dense as lead!)
It also has incredible optical properties, with an exceptionally high RI of 2.57 and extreme dispersion of 0.221 (diamond is 0.04!), which make it an amazing gem when faceted. Cut stones are exceptionally beautiful with a weirdly vivid, venomous color, high brilliance and play of color. Its only real downside is its hardness of 4.5 and brittleness which make it a definite collector's stone for display instead of for jewelry.
