This is a striking blue striped agate pendant, Marquise-shaped. It has a really interesting quartz-lined vug, or little "cave", visible in the upper left. This comes with a sterling silver bail and an 18" necklace.
This stone is indeed banded agate and quartz, but it has been dyed in order to obtain the unusual coloration. These come in many variations and are indeed striking.
Striped agate is a translucent, banded, often colorful variety of chalcedony (quartz). Very popular for both collectors and lovers of jewelry.