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 cavity. They originate from different points in the first agate layer lining the cavity and are microscopic, with identical optical properties and composition. The fibers can be revealed in a thin slice when illuminated from behind by a distant a point light source, in slices thinner than these, ideally thinner than 1 mm. The pattern of fibers can be seen in thin sections of roughly 30 microns (thousandths of a millimeter) when viewed in a polarizing petrographic microscope.
Students should see all the varieties of quartz. Agate is quartz, but in appearance is very different from the crystalline forms.
For a detailed treatise on agate, see the extraordinary Quartz Page by Amir C. Akhavan. Use this link, which takes you directly to agate: http://www.quartzpage.de/agate.html. Chrome chokes on this website because it isn't https. Safari opens it right up.
The Quartz Page is a beautifully organized website. Chapter topics are listed in an index at the top left of each page. It's all worth reading.
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