These are color change Holmium laser garnets (Ho:YAG), which are yellow in normal lighting and turn rich pink under fluorescent illumination. Each individual stone is labelled with its carat weight and letter in the photo and they can be purchased individually.
This is a nice material, holmium yag. It is used as a laser crystal for far infrared solid state lasers. In normal lighting it has a light yellow color, light enough to show the brilliant sparkle and dispersion of the YAG. Under fluorescent illumination it turns a rich pink. This is fun because it's the exact opposite of Nd+ YAG which is pink in normal illumination but turns colorless or light yellow under fluorescent lighting, allowing for a fun magic trick swapping the colors between them.
With a hardness of 8.5 it is ideal for use in all kinds of jewelry.