This is facet rough of light blue ytterbium YAG, a type of blue laser garnet. The color varies from light greyish-blue to more saturated blue-green.
This is an uncommon type of YAG, grown for use in some infrared lasers including ones used for high power engraving. The Yb gives it a beautiful blue-green 'poolwater' blue color very similar to paraiba tourmaline, and it shifts towards the blue or green ends of the color range depending on the light source. While it is used in a variety of infrared laser systems it is surprisingly hard to actually get as industrial scrap--this is because the material is usually annealed immediately after growth, which makes it suitable for use in lasers but also removes the color.
YAG is an amazing material for faceting - not only is it easy to cut and polish, but it also has a high hardness (8.5), a solid RI (1.83), and good dispersion. It is ideal for beginners as a learning material.
