This is 'zombie' garnet, a type of lab-created YAG laser garnet with an unknown impurity granting it an unusual yellow-green color. Some of these stones have minor inclusions.
This is an weird one, YAG doped with cerium and...something else? This material occurs at the tips of some boules of Ce+Nd-doped laser YAG. The variable yellow color and fluorescence all but guarantee it contains cerium, but the green color is very odd. The result is a very strange look, a swampy yellow-green with an odd glow that makes it look almost opaque in some lighting.
These stones also show unusual dual fluorescence, with a mix of bright yellow and pink under shortwave UV and yellow-green under both blue light and shortwave UV.
YAG has a high hardness of 8.25-8.5 and no durability issues. It is ideal for use in jewelry.
