Fluorite is considered to be one of most popular minerals in the world among gem and mineral collectors, second only to quartz.Due to the many color variations of fluorite, it can resemble a variety of gemstones; however, it can be easily distinguished by its relatively low level of hardness. Some possibilities for confusion include calcite, but calcite is slightly softer and has a different crystal form. Quartz is much harder and lacks the cleavage of fluorite, while apatite is slightly harder and has a different crystal form. Another distinguishing characteristic of fluorite is its property of thermoluminescence; the ability to glow when heated. One variety of fluorite known as 'chlorophane' demonstrates this well and can display thermoluminescence even when held in the hand. The thermoluminescence is green to blue-green.Fluorite has an attractive vitreous luster and gem-quality specimens should be free from inclusions visible to the eye. Gem grade fluorite typically occurs with excellent transparency, while lower-grade, industrial and commercial-use fluorite is often translucent.