Transparent clear base glass, with swirls of color on and under the surface.  The smaller marble has a strange dark sub-surface line (see photos), at first glance looks like a crack, but I think its a factory flaw.  The larger marble shows some nice, fizzy air bubbles and the colors are brown, yellow and white.  Smaller one's colors are orange, brown, white and pale blue.  5/8" and 3/4" in diameter.   We reference Marbles 5th Edition by R. Block, page 102. Listing price is for the two marbles.