Dogtooth and Nailhead Spar in Jewel Cave
Jewel Cave is currently the 3rd most extensive cave network in the world. It is believed to have formed completely underwater, thus leading to the extensive coating of calcite crystals.
On the righthand side of the image, some of this coating of calcite crystals can be seen. These crystals are made up of two primary types: dogtooth spar and nailhead spar.