Marcasite
Marcasite may not be one of the most memorable minerals, but still it has its beauty. The name marcasite comes from the Arabic word for a different mineral: pyrite. These two minerals are polymorphs, meaning that they share the same chemical formula - iron sulfide (FeS2) - however, marcasite is created with a different crystal structure. Besides, marcasite and pyrite often appear together and also associated with other minerals, such as galena, sphalerite, dolomite, calcite and fluorite. This mineral generally presents itself with metallic pale brass-yellow colored crystals and tabular shapes. On fresh surfaces, marcasite's color can vary to tin-white with a bright metallic lustre. Marcasite tarnishes to a dark brown brass-like color, but in some cases, it can become iridescent, which can be interesting for collectors who have a weakness for colored minerals.
Marcasite is mostly found in sedimentary deposits and low temperature ore veins with highly acidic conditions.
An interesting thing is that marcasite's flatenned tabular crystals often join together in groups - called "cockscombs" - which gives this mineral a distinguished appearence.
Due to its unstable crystal structure, marcasite can easily crumble and break up. Also, when it undergoes oxidation, disintegrates in melanterite, which contains sulfuric acid, so you should definitely wash your hands after you handle marcasite that is altering.
The uses of this mineral have not always been the same: during the Middle Ages it was used for medicines and in the Victorian era marcasite was quite popular in the making of jewlery. Even today it is used to create, for example, bracelets or rings. Perhaps some of you ladies are wearing marcasite right now and you have no idea!
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Photo credits: http://bit.ly/2rWgcFG - photos by Harjo Neutkens http://bit.ly/2r2UFwS