Rock crystal is the name given to high-clarity, transparent crystals of colorless quartz, which are popular with mineral collectors as well as occasionally being fashioned into faceted or cabochon-cut gemstones. Although rock crystal specimens are relatively common, their collectability arises from the vast number of interesting forms taken by individual crystals and/or inter-grown crystal formations. These include the small, doubly-terminated so-called “Herkimer Diamonds” and near-perpendicular Japan twins, as well as large crystal-lined geodes.

In addition, rock crystal specimens are also highly collectable when they contain inclusions that are of visual interest, and the trade has provided the most prevalent of these with sub-variety names that are based on the appearance (e.g. dendritic quartz, medusa quartz, etc.) or identity (e.g. tourmalinated quartz, rutilated quartz, hematoid quartz, etc.) of the inclusions.