This is a striking blue, semi-translucent, Banded Agate cabochon, oval-shaped. The photo does not do justice to the translucent, 3-dimensional appearance of the stone. Banded Agate is one of the most attractive forms of pendant beads!
This stone is indeed agate, but it has been dyed in order to obtain the unusual coloration. These come in many variations and are indeed striking. Although this is a cab, you can also glue on "bails", or have the piece drilled to accept a pinch bail. Either approach will let you turn this sample into a fine pendant.