Siderite is a common mineral belonging to the Calcite group of minerals. Siderite and Calcite are isomorphous with one another (Isomorphous: Having the the same molecular arrangement and crystal form but with different specific elements), thus having similar crystal forms. Its color is quite varied, including light to dark brown, yellow, yellow-brown, gray, green, red and white. Its luster is vitreous, silky or pearly, and occasionally exhibits iridescence. This unique mineral is associated with numerous aesthetic minerals, including Quartz, Calcite, Pyrite, Sphalerite, Fluorite and Barite.
The crystal system for Siderite is trigonal, commonly forming coxcomb and platy aggregates of curved rhombohedrons that are flattened to appear bladed. Other forms include botryoidal, stalactitic, grainy, radial, fibrous, nodular and massive.
Siderite is a common mineral found worldwide. Notable localities for quality specimens include Bolivia, Brazil, China, Germany, Portugal, Romania and the USA (Arizona, Colorado).