Quartz is one of the most common of all rock-forming minerals, second only to feldspar in abundance in the Earth's crust. Humans have been intrigued by quartz, which often occurs as hexagonal crystals, since the early days of the Greeks. The often beautiful and economically important mineral has been widely utilized around the world as a gemstone, appearing in a wide array of colors, such as pink, purple, and yellow, in addition to its familiar water-clear form, which can be seen in the faceted pendant above. Quartz has also been frequently used for more practical purposes, including the production of glass and ceramics.