Chocolate dogtooth calcite, small

Locale: Santa Eulalia, Chihuahua, Mexico

Approx size: 2–2.5"

Approx weight: 45–120g

Known by the trade name "chocolate dogtooth calcite," this type of calcite from the famed Santa Eulalia mining district grows in hedgehog clusters of sharp dogtooth (scalenohedral) crystals. The variety gets its distinctive opaque dark brown appearance from iron oxides; a layer of hematite-included calcite is deposited on the surface during a later phase of growth.

Broken crystals reveal the first phase of growth was quite clear, a semi-transparent interior with a dark brown border. Calcite has perfect cleavage so it is common to have a point or two that has cleaved off, leaving a flat smooth window displaying this growth. Many specimens also show signs of natural etching that can leave the tip of crystals blunted or hollow. Some have a neat almost hoppered effect!

Very small crystals often dot the surface of the calcite, as seen in the initial surface detail image (featuring Option C). This includes tiny points of quartz, some with a slight smoky-quartz tint. The lumpy brown mineral on option H may be weathered goethite.

UV reactivity: the presence of iron dampens any reaction in the brown calcite, which is non-reactive. Where the clear calcite interior is exposed you may get a good peachy-red reaction using 365nm UV light, with some phosphorescence. All options have good reaction, except Option G which has limited reaction and less-visible white reaction. 

Option D has especially good zoned fluorescence (it is the example shown fluorescing!). It has three flower-shapes of yellow glow at the center of radial calcite formations, with peachy-red material around it.