Gem library
Rock crystal
Clear quartz, the archetype of crystal
Rock crystal is pure, colourless, transparent quartz — the stone that gave the word crystal its name. It is also the most studied mineral on the planet: its piezoelectricity beats at the heart of every quartz watch and all modern electronics.
- Hardness (Mohs)
- 7
- Density
- 2.65
- Crystal system
- Trigonal
- Refractive index
- 1.544 – 1.553
- Cleavage
- None
Mineralogical identity
- Family
- Quartz — silica group
- Species
- Quartz
- Variety
- Hyaline quartz
- Chemical formula
- SiO₂
- Crystal system
- Trigonal (rhombohedral)
- Classification
- Semi-precious stone
- Rarity
- Very common
- Indicative price
- €0.30 – 5/ct
Etymology
From the Greek krustallos, «ice»: the ancients believed it was water frozen forever by the cold of the mountain tops.
Physical & optical properties
- Hardness (Mohs)
- 7
- Density
- 2.65
- Refractive index
- 1.544 – 1.553
- Birefringence
- 0.009
- Cleavage
- None
- Fracture
- Conchoidal
- Lustre
- Vitreous
- Diaphaneity
- Transparent
- UV fluorescence
- None
- Pleochroism
- None
- Colours
- colourless, white
Formation, geology & history
The perfect crystal
Hyaline quartz crystallises as hexagonal prisms capped by a six-faced pyramid, a geometry so regular that it served as a model for early crystallography. The faces often carry characteristic horizontal striations, a signature of the growth mode. Alpine fissure crystals, grown slowly in the cracks of granite massifs, differ from Brazilian geode crystals, which are shorter and stouter. Some pieces enclose spectacular inclusions: golden rutile needles (Venus hair quartz), black tourmaline, green chlorite, or even a bubble of water and gas millions of years old (enhydro quartz).
Piezoelectricity and industry
Compressed, quartz generates a voltage; fed with alternating current, it vibrates at a remarkably stable frequency. This property, discovered by Pierre and Jacques Curie in 1880, underpins quartz timekeeping, radio filters and the oscillators in every processor. Because industrial demand is colossal, quartz has been grown in autoclaves since the 1950s: world synthetic production far exceeds mining output. Natural rock crystal is now mostly destined for collections, crystal healing and optics.
History and art
The Romans carved cups and seals from rock crystal, then a more prestigious material than glass. The treasury of Saint-Denis, the Fatimid crystals of Cairo, Mesoamerican skulls and European scrying spheres all testify to a continuous fascination. Alpine crystal hunters — the «cristalliers» — still form a regulated profession in Switzerland and Savoy, with their codes, their sectors and their legendary finds, such as the Planggenstock pocket discovered in 2005.
How to recognise it
- BirefringencePlace a transparent crystal over a thin line: quartz very slightly doubles the image. Glass, being isotropic, never doubles anything.
- Striations on prism facesHorizontal striations perpendicular to the axis are an almost infallible marker of natural quartz.
- Lasting coolnessQuartz conducts heat away: it stays cool for several seconds against the lip, where glass warms at once.
Imitations, treatments & confusions
- Lead glass (Bohemian crystal)No mineralogical relation: it is glass, hardness 5 to 6, often moulded. It warms quickly in the hand and does not double lines.
- Synthetic quartzChemically identical, mostly used in electronics and sometimes sold as over-perfect points. A total absence of inclusions and unnaturally clean faces are the signals.
- Glued or ground pointsSome «natural points» are in fact ground from massive quartz: over-sharp edges, no striations, a flat sawn base.
Deposits & origins
Alps (Switzerland, Mont Blanc massif — the famous Alpine fissure crystals), Brazil, Madagascar, Arkansas (USA), Himalaya, China, Namibia, France (Alps, Auvergne)
Care & precautions
- Insensitive to water, salt and sun: the easiest stone of all to look after.
- Warm soapy water and a soft brush; ultrasonic cleaning is only a problem on cracked pieces.
- Fine points snap on impact: store collection crystals in a compartmented box.
- Regular dusting with a soft brush preserves the brilliance of geodes and druses.
Traditions & crystal healing
- Chakras
- Crown · all
- Zodiac signs
- All signs
- Element
- Ether
- Month
- April (secondary)
Tradition describes it as an amplifier: it is said to reinforce the effect attributed to other stones and clarify intention. That is why it sits at the centre of most arrangements, often as a single point on a desk or a druse in a living room.
The virtues described here belong to tradition and cultural heritage. They are in no way medical advice and never replace a consultation.
Frequently asked questions
Rock crystal or glass?
Three simple tests: quartz stays cool, scratches glass, and slightly doubles a fine line seen through it. Glass fails all three.
What is a phantom quartz?
A crystal whose growth stopped then resumed, leaving inside the ghostly silhouette of the earlier crystal, outlined by a thin layer of chlorite or hematite.
Is rock crystal valuable?
As a gem, very little. As a collector's specimen, enormously: an exceptional Alpine pocket or a quartz with rare inclusions can reach tens of thousands of euros.