Ericssonite
Ericssonite has a general formula of BaMn2FeO[Si2O7](OH). It was discovered in 1967 and named after John Ericsson (July 31, 1803 – March 8, 1889), a well known Swedish American inventor, engineer and designer of the iron-clad ship USS Monitor. Ericssonite was discovered in the Jakobsberg Mine in Värmland, Sweden. Ericcsonite is monoclinic; this means it contains three unequal vectors, two of these vectors angles are perpendicular while the other is at an angle greater than 90°. When talking about its optical properties, ericssonite is anisotropic which means that the mineral has more than 1 index of refraction, causing light to vary in speed depending on which axis it is traveling through. The value of relief, the way the mineral appears to stand out when viewing it in plane polarized(PP) light under a microscope, in this mineral ranges anywhere from 1.802-1.891. This means that light travels anywhere from 1.802-1.891 times as fast in this mineral as it does in air, assuming the index of refraction of light in a vacuum is one. Since ericssonite is monoclinic, containing three unequal vectors, it is no surprise that it has three index of refraction, which is just the measurement of the speed of light in the mineral compared to the speed of light in a vacuum. Lastly when looking at ericssonite in PP light it is usually seen as a deep reddish-black. Ericssonite is only found in the Langban mine in Sweden, and usually in a metamorphic manganese ore body. Also it is always inter-grown with orthoericssonite, which is almost identical to ericssonite except you will find an extra silicon and oxygen in its chemical formula. Besides the fact that it is magnetic, the most unusual aspect of ericssonite is that it is just a very rare mineral, and it is only found in one place in the world, making it most useful for a collector of rare minerals.