These are faceted lab created rutile, titanium dioxide. Each is labelled with its weight and letter in the photo and they can be purchased individually.
Note that X has an inclusion and is sharply discounted.
This is rutile, and it has the highest dispersion of any transparent gem material. In practical terms that means it has the most insane fire you will ever see in a gemstone, throwing rainbows with the slightest provocation. It is simply impossible to capture on camera--they look good in a picture but great in person. It only has two downsides: its relatively low hardness of 6.5, a bit over tanzanite and suitable for pendant usage, and how difficult it is to get these days.
Crystals of rutile are grown for a few high end optical applications, particularly for very powerful prisms, as well as for use as a substrate to grow other crystals on. Production is fairly scarce today and often what we can get is of very low quality, so much of what we get is from old stock. Some rutile is golden, some light yellow and it can very occasionally be a lighter blue or blue-yellow bicolor--all of them are absolutely stunning once cut.