Pure quartz is a transparent mineral. Quartz is usually almost pure SiO2 but even very small amounts of other elements or mineral inclusions can seriously alter the color. One of the strongest coloring agents is mineral hematite. Very fine-grained hematite pigment often covers sand grains in a sandstone. But hematite may also form inclusions inside the crystals.
There are many names given to the varieties of quartz. One of them is “eisenkiesel” which is not nearly as well known as jasper, agate, onyx, etc. This is red quartz which is strongly colored because of hematite inclusions. Sometimes quartz crystals that contain other iron-bearing oxides are named the same way. This term obviously has its origin in the German language but the beautiful crystals I want to demonstrate today come from Spain. They were known as Jacinto de Compostela quartz crystals already centuries ago. These crystals grow in a gypsum-bearing marl deposit which makes them somewhat similar to the Pecos Diamonds from New Mexico that grew in a gypsum deposit.
Jacinto de Compostela quartz crystals from Spain. The width of the view is 40 mm.