Agate Bookends as shown. You'll agree that agates are beautiful, showing waves or bands of colors. Some pieces include crystal clusters. Agates are banded varieties of cryptocrystalline quartz (silica), generally showing two or more colors. It comes in brown, grey, bluish tones and varieties such as turritella, dendritic, lace and moss. Cryptocrystalline is a rock texture made up of such minute crystals that its crystalline nature is difficult if not impossible to see, even microscopically using polarized light.