Amazonite

Amazonite is the blue-green variety of microcline feldspar. For decades, the cause of its color was debated - lead impurities, iron, water content, and structural effects were all proposed. Research in the 2010s established that the color is caused by small amounts of lead and water within the crystal structure, combined with natural radiation that creates the specific color centers responsible for the blue-green hue.

Amazonite forms in granitic pegmatites - the coarse-grained igneous rocks that crystallize from the last volatile-rich fluids of cooling magma. The classic locality at Pikes Peak, Colorado, produces stunning specimens of bright green amazonite crystals perched on gray granite matrix, often accompanied by smoky quartz and sometimes albite. These specimens are among the most photogenic in American mineralogy.

Despite its name, amazonite has no confirmed connection to the Amazon River. The name may come from green stones traded by indigenous peoples along the Amazon, but those stones were likely nephrite or other green minerals, not amazonite.

Amazonite is identified by its distinctive blue-green color in a feldspar mineral. No other common feldspar is this color. It has two directions of cleavage at nearly 90 degrees (characteristic of feldspar), hardness 6, and often shows a grid-like texture called perthitic exsolution (fine white streaks within the green).

Distinguish from turquoise (opaque, no cleavage, different luster), chrysoprase (chalcedony, waxy luster, no cleavage), and green aventurine (shows sparkly aventurescence). Amazonite's cleavage surfaces and the fine white streaking are diagnostic when present.