Citrine
Yellow in color, citrine is a type of quartzite crystal glass. Scoring a 7 on the Mohs scale, which measures a mineral's overall hardness, the gemstone is hard and typically resistant to scratches. Citrine is the birthstone for the month of November and its yellowish hues reflect the fall season. The most sought-after citrine gems have a clear and bright color. Not typically found in nature in the United States, citrine is readily found throughout Europe, particularly in Spain and Hungary.
Smoky Quartz
Smoky quartz is a gem rich and dark in color. True to its smoky name, it is a transparent gem that is typically dark brown or black. Heating of quartz allows the quartz to metamorphose, which under the correct conditions leads to the production of smoky quartz. Due to the way it is produced, hexagonal striations are produced within the stone. A 7 on the Mohs scale, smoky quartz has been used as a gemstone for centuries.
Prasiolite
The prasiolite quartzite glass gemstone is a light greenish hue. Also scoring a 7 on the Mohs scale, prasiolite is a strong and solid crystal glass. While not often found readily in nature, it has typically been found in parts of Canada, Brazil and Poland. Prasiolite is a rare type of quartzite glass that most often occurs as a heat-treated amethyst. It is most often found as part of gemstones in jewelry.