This is Samarium-doped YAG, a lab created garnet with a light yellow to yellow-orange base color and bright orange to orange-pink fluorescence.
Sm:YAG is a rare lab-grown garnet which has a few niche uses--in particular it is used for measuring pressure in experiments with diamond anvils because the color of its fluorescence changes depending on how much pressure it is under. It is not commonly grown so we had this crystal grown for us by one of the companies we work with as an experiment. Because they had never worked with samarium before there was a minor hiccup during growth, so half of the crystal ended up with more samarium in the bottom half, giving a slightly richer orange-yellow base color and a pinker fluorescence. Both types will cut amazing gems!
YAG in general is an ideal material for faceting, which a high hardness of 8.5, no cleavage, no problems with polishing and excellent optical properties with an RI of 1.82. It is considered one of the easiest materials to facet.
