A highly collectible lab-created colour-change Yttrium Aluminium Garnet.

While YAG is not an uncommon gem material (it has been commercially available since the 1960's) this exceptionally scarce example is the result of  Russian experimentation during the 1950's.
The producers grew a limited number of crystals with a colour-change effect while experimenting with colouring dopants. 

This well-cut gem appears orange-pinkish-red in daylight (images 1 to 4), a colour similar to a Padparadscha Sapphire, and reverts to an orange-brown colour in incandescent light (Images 5 to 8).

weight:  7.00 carats
shape:   Fancy pentagon cut (very small table facet)
dimensions:   9.96 x 9.70 x 7.49 mm
treatment:  None