Mineral necessarily constant for all sides, which is a function of its structure; crystallographic weakness renders some directions softer than others. An example of this property exists in kyanite, which has a Mohs hardness of 5½ parallel to [001] but 7 parallel to [100]. The most common scale of measurement is the ordinal Mohs hardness scale. Defined by ten indicators, a mineral with a higher index scratches those below it. The scale ranges from talc, a phyllosilicate, to diamond, a carbon polymorph that is the hardest natural material. The scale is provided below: Lustre indicates how light reflects from the mineral's
Selenite (mineral) Mine's Cave of the Crystals being 12 metres long and weighing 55 tons. The etymology of selenite is through Middle English "selenite", from Latin "selenites", from Greek "selēnitēs (lithos)," literally, moonstone or stone of the moon, from selēnē (Moon). The ancients had a belief that certain transparent crystals waxed and waned with the moon. From the 15th century, "selenite" has referred specifically to the variety of gypsum that occurs in transparent crystals or crystalline masses. All varieties of gypsum are very soft minerals (hardness: 2 on Mohs Scale). This is the most important identifying characteristic of gypsum, as any variety