Specimen: TMM 3363
Mineral: Calcite (scalenohedrons)
Collector: Glen Evans

Specimen: TMM 3363
Mineral: Calcite (rhombohedral crystal)
Collector: Glen Evans

Calcite (calcium carbonate)

This example is a stalactite from a cave in Texas.  The calcite is precipitated from ground water as that water emerges into caverns.

Specimen is part of the Museum Mineral Education collection.

Image is about only one third of the original stalactite measuring about 2 feet in length.

Quartz

Specimen: TMM 3379
Mineral: Amethyst (Quartz) 
Locality: Arizona
Collector: Glen Evans

Specimen is about 4"x6"

Quartz (silicon dioxide) is colorless, but occurs in nature in many colors! Violet colored quartz is called amethyst, and the color may result from impurities of iron in the crystals. Smokey quartz, yellow-brown to black in color, is attributed to free silicon formed by exposure to radiation.