These are BGO-20 aka Bismuth Germanate specimens. Some are end cuts that show crystal faces while others have been sawn into flat plates in the process of making wafers for scientific use.
Bismuth Germanate (in this case, specifically Bi12GeO20 aka sillenite) is a very unusual lab created crystal. It is valued in science for its highly unusual electrooptical properties which allow it to be used in nonlinear optics and for acousto-optical modulation. It has exceptionally good normal optical properties too, with an RI of 2.55 and dispersion of 0.221(!! diamond is only 0.04) giving it incredible brilliance and fire. On top of that this BGO-20 has an incredibly high density of 9.2, about 80% of metallic lead(!!!!). It's overall a super cool synthetic very rarely seen in the gem world. Note it does have a relatively low hardness of 4.5.
