A macrocrystalline Quartz.  Clear Crystal with minor inclusions.
 
Rock Crystal is a form of Quartz that has grown without the presence of other minerals or impurities, creating a completely clear stone. Lower grades may contain a milky or clouded effect throughout its matrix. It may feature a patterning similar to suspended ‘smashed glass’, which can sometimes have the ability reflect light as a rainbow.
Due to its excellent ability to dispel heat and remain very cold for long periods of time, Rock Crystal was initially thought to be a magical form of ice that will always remain frozen. For this reason, it has been used alongside treatments for fever to soothe high temperatures, providing comfort and relief from otherwise distressing symptoms. It is known to have been used in healing practices throughout several cultures and Ancient Civilisations (such as Roman, Aztec, Native American and Ancient Egyptian), particularly when trying to diagnose a illness or disorder.
Rock Crystal is a very common crystal found in healing, and is integral to a range of treatments.