This is a delicate, blue & black, semi-translucent, shield-shaped Dragon's Veins Agate pendant. It comes with the sterling bail and the 18" black necklace. The photo does not do the 3-dimensional appearance of the stone justice.
These "Dragon's Veins Agates" are indeed quartz or in this case, banded agate, but the stones have been heat treated, bleached, and then dyed. They come in hundreds of variations and are indeed striking.
(As always, if you don't like the choice of bail or necklace, just contact me and we can make changes.)