One of the most intriguingly named is dogtooth calcite. Not, in fact, a fossil of actual canine teeth, “dogtooth” is actually a reference to the shape of the crystals: the golden honey-colored calcite naturally forms into very pointy pyramid shapes. At times, the crystals will double terminate, creating beautifully faceted footballs.
Calcite is a soft mineral that is easily scratched with a knife. Found in more different shapes than any other mineral, calcite is sedimentary and has perfect cleavage. In addition to the desired golden honey of the dogtooth crystals, calcite also comes in clear, black, grey, white, blue, pink, brown, yellow and red. Both fluorescent and phosphorescent, calcite is also the mineral that has preserved trilobites’ detailed compound eyes for us to admire.
Iceland spar, another type of Montana calcite, forms in a rhombohedrum shape (picture a box that has been slightly squished and tilted to one side). The most sought after of this type is clear and referred to as optical calcite. This calcite is strongly double refractive, meaning that if a square-ish piece of optical calcite is laid on top of a straight line or image, the viewer will see two separate images. This happens because the calcite splits one ray of light into two separate rays. One remains stationary, while the other can revolve around the stationary ray. As the piece of calcite is turned a quarter turn, the lines will recombine into one line; another quarter turn, and the two lines reappear. Besides being entertaining, this capability made optical calcite valuable in several applications: In World War II, it was used in gun sights and anti-aircraft weaponry; before Polaroid cameras, it was used in polarizing microscopes; and it was also utilized in experiments to refract light in ways that appear to produce a “cloak of invisibility.”