What is quartz? The most common gem mineral and why it still matters

Chemistry and structure

Quartz crystallises in the trigonal system (a subdivision of the hexagonal system). Its crystal structure consists of silicon atoms each bonded to four oxygen atoms in a continuous three-dimensional framework: the classic SiO2 framework silicate structure. This structure is extremely stable, which explains quartz's resistance to chemical weathering and its persistence in river sediments and sandstones long after other minerals have decomposed (GIA Gem Reference Guide, 2006, pp. 90-91; Klein, C., Manual of Mineral Science, 2002).

Physical properties shared across all macrocrystalline quartz varieties: Mohs hardness 7 (harder than most glass but softer than corundum and beryl); no cleavage (conchoidal fracture); specific gravity approximately 2.65; refractive index approximately 1.544-1.553 (low, contributing to quartz's relatively modest brilliance compared to higher-RI gems). The combination of hardness 7 and no cleavage makes quartz durable enough for most jewellery applications, though it scratches from common abrasives like dust (which contains silica) more readily than corundum (GIA; Klein, 2002).

The six macrocrystalline quartz gem varieties, all SiO2 and all Mohs 7. Colour is produced by different trace elements and irradiation conditions in the crystal. Most commercial quartz has been heat treated or irradiated to improve or standardise colour. Source: GIA Gem Reference Guide (2006).

Amethyst: the purple variety

Amethyst is purple quartz coloured by iron (Fe4+) in specific coordination within the crystal, combined with natural irradiation during crystal growth. The purple is produced by colour centres formed when iron substitutes for silicon and natural gamma radiation from surrounding rocks causes specific electron configurations in the iron sites. The colour intensity varies from pale lilac to deep vivid purple depending on iron concentration and irradiation history (GIA Gem Reference Guide, 2006, pp. 90-92; Nassau, K., 1978).

The finest amethyst colour is a vivid, strongly saturated purple, described in the trade as "Siberian" after the historically prized dark purple material from the Ural Mountains of Russia. Today "Siberian colour" is a quality descriptor, not necessarily an origin designation: amethyst from Brazil, Uruguay, Zambia, and Madagascar can achieve equivalent colour. The depth and evenness of the purple and the absence of brownish or greyish modifiers define fine amethyst (GIA; Wise, 2016, pp. 291-295).

Major sources: Brazil (Minas Gerais, Rio Grande do Sul; highest volume), Uruguay (particularly vivid purple, often smaller crystals), Zambia (fine deep colour), Madagascar, and Russia (Ural, historic Siberian colour). Brazil dominates commercial production by volume.

The primary quality issue in commercial amethyst: colour zoning. Many amethyst crystals show uneven colour distribution, with deeper purple at the crystal tips and paler colour toward the base. Skilled cutting can position the table facet to maximise the apparent colour depth, but heavily zoned rough produces stones that appear pale when viewed off-angle. Fine amethyst with even colour throughout commands a premium over commercial zoned material (GIA; Wise, 2016).

Citrine: yellow to orange quartz

Natural citrine is yellow to orange quartz coloured by Fe3+ (ferric iron) in a different configuration from amethyst's iron colour centres. The warm yellow to orange colour of natural citrine is less common than amethyst; most commercial "citrine" is actually heat-treated amethyst, which converts the purple Fe4+ colour centres to yellow/orange Fe3+ colour centres when heated to approximately 470-560°C. The resulting material is commercially indistinguishable from natural citrine to visual examination, and the treatment is universally accepted and undisclosed in the citrine trade (GIA; Wise, 2016, pp. 295-298).

Natural citrine from Brazil (particularly the Palmeira citrine with its pale yellow colour and the Madeira citrine with its rich orange-red) is valued more highly than heated amethyst, but the distinction is difficult to establish without advanced laboratory examination. For commercial citrine jewellery purposes, the origin of colour is not a practical concern. For collector-grade large crystals, natural citrine carries a premium (GIA; Wise, 2016).

Rose quartz: the pink variety

Rose quartz is pale pink to vivid pink quartz coloured by a complex mechanism involving titanium, manganese, and aluminium in combination with irradiation. Unlike most other coloured quartz varieties, rose quartz colour is typically distributed throughout the crystal mass rather than in zones, because the colour mechanism involves microscopic fibrous inclusions of a titanium-rich mineral distributed through the quartz matrix rather than discrete colour centres. This is also why rose quartz is almost always translucent rather than transparent (GIA Gem Reference Guide, 2006, pp. 92-93; Wise, 2016).

Fine rose quartz with the deepest, most vivid pink and minimal cloudiness or milkiness commands modest premiums. Very large, very clean rose quartz is unusual and valued by collectors. The most significant commercial use of fine rose quartz is in large carvings and spheres rather than faceted gems. Brazil (Minas Gerais) and Madagascar are primary sources.

Smoky quartz

Smoky quartz is brown to near-black quartz. The colour is produced when aluminium (Al3+) substitutes for silicon in the crystal structure and natural irradiation (or laboratory irradiation) creates specific colour centres. Natural smoky quartz occurs globally; irradiated colourless quartz can be converted to smoky at low cost. The distinction between natural and irradiated smoky quartz is commercially irrelevant at most price points because smoky quartz is an affordable commercial gem. Fine deep "morion" (near-black smoky quartz) in large sizes is the most valued form but still accessible in price (GIA; Wise, 2016, pp. 298-300).

Treatment prevalence in quartz

Quartz is one of the most treated gem species in the commercial market. The treatments are accepted and undisclosed in most commercial contexts because the treatment does not significantly affect durability and the resulting colour is indistinguishable from natural colour to visual examination:

Heat treatment of amethyst to produce citrine: standard, undisclosed. Irradiation to produce smoky quartz or to deepen amethyst: standard, undisclosed. Heat treatment to produce prasiolite (green quartz) from specific amethyst: uncommon, less standardised. Dyeing of chalcedony and microcrystalline quartz: exists and should be disclosed but often is not at commercial price points.

The practical consequence: at commercial quartz prices (USD 5-50 per carat for most varieties), the treatment question is not commercially significant. For large high-quality amethyst above USD 500 per carat, natural Siberian-colour origin with a certificate is meaningful (GIA; AGTA; Wise, 2016).

Quartz in Jyotish tradition

Sphatik (rock crystal, colourless quartz) is used in Jyotish and Hindu religious practice as the crystal mala (prayer beads) and as Sphatik Shivlings and other sacred objects. It is associated with purity and is used in puja rituals. Rock crystal sphatik is not a planetary stone in the Navratna system but has its own Jyotish and religious significance independent of the gemstone tradition.

Amethyst is sometimes recommended by contemporary Jyotish practitioners as an alternative stone for Saturn (Shani) based on colour similarity to blue sapphire, but this is a modern extension not supported by classical texts, which specify natural blue corundum (Neelam) for Saturn. Confirm with your practitioner (Behari, 1991; Johari, 1986).