Amygdaloidal obsidian is the matrix for an open "egg" of light gray, cristobalite (a tetragonal polymorph of quartz) which lines the inside. Perched aesthetically inside the cristobalite vug is a gemmy, yellow-green crystal, 0.2 cm across, of fayalite. This is an end member of the olivine group, of which, the most famous member is forsterite var. peridot. But you even more seldom see nice sharp (gemmy!) fayalites.