agate - polished slice from an agate-filled geode from Brazil

Agate is a cryptocrystalline variety of quartz - essentially a translucent banded multicolored chalcedony. The colors are caused by impurities - pure agate is white, gray or blue-gray. The banding in this agate is primarily caused by variations in translucency of the concentric layers that follow the wall of the cavity.  Normally, agates like these are shades of brown and gray, but blue and green also occur, though not as commonly. Agate is easily dyed, creating slices with an unfortunate rainbow of artificial greens, blues and reds. These slices are natural. In rocks like basalt, which is poor in silica, gas bubble cavities in the solidified lava are gradually filled with a colloidal watery solution of silica picked up from weathered silicates by descending rainwater. Brazilian agates, like these, typically form in cavities in basalt flows and show weak banding but good translucency. Surprisingly, the agate is composed of normally invisible fibers radiating in from the edge of the cav