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How amethyst cathedrals form

Cordua, William S. (William Sinclair), 1947- (St. Croix Rockhounds, Stillwater, MN, 2007)

Why pyrite crystals have different shapes

Cordua, William S. (William Sinclair), 1947- (St. Croix Rockhounds, Stillwater, MN, 1994)

Why do such large crystals form in pegmatites?

Cordua, William S. (William Sinclair), 1947- (St. Croix Rockhounds, Stillwater, MN, 1987)