A very rarely encountered lab-created Bismuth Germanate gemstone.

Bismuth Germanate, also known as "Sillenite",  has no natural counterpart and was originally created for use as a detector/ scintillator material in the optics industry due to its unusual electro-optical properties.
It is a Bismuth Germanium Oxide (BGO) with chemical formula Bi₄Ge₃O_12.

As a gem material it has some notable properties too.
It has a very bright lustre, a high Refractive Index of 2.55 and most obvious to the eye, a dispersion (fire) of 0.221, roughly 5 times that of Diamond at 0.044!

This results in a rather attractive display with the stone in hand.
Still images do not do this gem justice, so please take a look at its fiery appearance on this short YouTube video :   https://youtu.be/btMjRHYNLIE

weight:  11.78 carats
shape:   Modified octagon cut
dimensions:   10.32 x 7.86 x 5.94 mm
treatment:  Not applicable