These are faceted terbium gallium garnet aka TGG. Each is labelled with its carat weight and letter in the photo and they can be purchased individually.
TGG is a very cool synthetic material. It has the same structure as a natural garnet, but has a composition that would never be possible in nature. The result is a bright and very dispersive gem with optical properties similar to diamond (cubic crystal structure with a lower RI but the same dispersion as diamond), giving it incredible brilliance and fire. It is not, however, grown for gem usage at all, but because of its incredible ability to act as a Faraday rotator. Light passing through TGG in the presence of a magnetic field has its polarization rotated, which might not sound that interesting but is incredible useful for lasers. Lasers shoot out powerful beams but are very fragile, and if any of the light bounces back at them it can quickly burn them out. With TGG it is possible to make a high-efficiency optical isolator that prevents any of that sort of backsplash. This is likely the first time it has ever been faceted as a gem, and if not it is close.
It also has bright green fluorescence under 365nm UV!
With a hardness of about 8 on the Moh's scale it is also an excellent material for use in jewelry.
