The chalcedony group: microcrystalline quartz
Chalcedony is the umbrella term for microcrystalline and cryptocrystalline quartz varieties where the crystal grain size is too small to see with the naked eye or even under a standard loupe. The individual quartz crystals are typically less than 1 micrometre in diameter, packed so densely that the material appears uniformly waxy-translucent to opaque rather than transparent. The chalcedony group includes:
Plain chalcedony (blue-grey, translucent), chrysoprase (apple green), carnelian (orange-red), sard (brownish red), bloodstone (dark green with red spots), agate (banded multiple colours), onyx (banded black and white), sardonyx (banded red-brown and white), jasper (opaque, multi-coloured), and several others. All are SiO2 with Mohs hardness 6.5-7 (GIA Gem Reference Guide, 2006, pp. 90-95; Wise, 2016).
Chalcedony group quality overview. Chrysoprase (apple green, nickel colour) is the highest-value chalcedony. Carnelian and plain blue chalcedony are commercial-tier staples. Agate is widely used but frequently dyed; disclosure varies. Source: GIA (2006).
Chrysoprase: the finest chalcedony
Chrysoprase is apple green to vivid green chalcedony coloured by nickel (Ni) rather than chromium or vanadium, making it unusual among green gems. The colour ranges from pale apple green to vivid emerald-like green at the finest quality. Fine chrysoprase was mistaken for jadeite in European gem collections before the 18th century; Frederick the Great of Prussia used chrysoprase extensively in the decoration of the Sanssouci Palace in Potsdam, where it was set alongside gold in floors, tabletops, and wall panels (GIA; Wise, 2016, pp. 300-302).
Major chrysoprase sources: Szklary, Poland (the historical European source, now largely depleted), Queensland, Australia (currently the primary commercial source), Tanzania, and Brazil. Fine Australian chrysoprase at its most vivid green commands USD 20-80 per carat, making it one of the more expensive chalcedony varieties (GIA; Wise, 2016).
Carnelian and sard
Carnelian is orange to reddish-orange chalcedony coloured by iron oxide (Fe2O3). Sard is the browner, darker variety. Both have been used for seal engraving since Mesopotamian antiquity, valued for their hardness relative to organic seal materials and their ability to take fine detail in intaglio carving. Commercial carnelian is frequently heat-treated (which converts iron hydroxides to oxides, deepening and standardising the orange colour) and sometimes dyed. The treatment is accepted at commercial prices. Most commercial "carnelian" in the market is heat-treated and/or dyed chalcedony from India, Brazil, or Madagascar (GIA; Wise, 2016).
Agate and onyx: the banded varieties
Agate is banded chalcedony showing alternating concentric or straight bands of different colours and translucencies. The banding is produced by rhythmic precipitation of silica from hydrothermal solutions in void spaces within host rock. Natural agate colours are typically white, grey, blue, and brown; the vivid colours of commercial agate (vivid blue, green, red, purple) are almost invariably produced by dyeing. The dye penetrates the more porous bands selectively, producing the coloured appearance. Dyed agate is commercially standard and is not a fraud when priced at commercial agate levels, but should be disclosed (GIA; Wise, 2016).
Onyx specifically refers to banded chalcedony with black and white parallel bands; sardonyx has red-brown and white bands. Both are used for cameo carving because the banding allows the carver to work in the upper layer while the contrasting lower layer provides the background. Natural onyx with well-defined bands is suitable for fine cameo work; most commercial black "onyx" used in modern jewellery is dyed black chalcedony (GIA; Wise, 2016).
Ametrine: the bi-colour rarity
Ametrine is a naturally bi-colour quartz showing both amethyst purple and citrine yellow within the same crystal. The two colour zones coexist because the crystal grew with different temperature and chemistry conditions in different sections, producing iron colour centres that respond differently to natural irradiation. The only significant commercial source of ametrine is the Anahi mine in the Pantanal region of eastern Bolivia; specimens showing the bi-colour character authentically are inherently from this deposit. (Heat-treated material converts uniformly to citrine, eliminating the bi-colour, so ametrine that appears bi-colour is either natural or separately dyed.) Ametrine is cut to display both colour zones proportionally, typically in emerald cuts or large fantasy cuts. It is affordable and distinctive (GIA; Wise, 2016, pp. 302-304).
Tiger's eye: chatoyant quartz
Tiger's eye is quartz pseudomorphed after crocidolite asbestos fibres: the original fibrous mineral has been replaced by quartz while retaining the fibrous structure, producing a chatoyant (cat's eye) effect when the parallel fibres reflect light as a moving band. The golden-brown colour comes from iron oxide staining of the silica fibres. The chatoyancy in fine tiger's eye produces a sharp, well-defined moving light line visible as the stone is rocked. Tiger's eye is found primarily in South Africa (Northern Cape province), where it occurs in quantities sufficient to supply the global market at low commercial prices. Blue tiger's eye (hawk's eye) is crocidolite that has been replaced by quartz without the iron oxidation, retaining a grey-blue colour (GIA; Wise, 2016).
Aventurine: the sparkling quartz
Aventurine is quartz with included microscopic platelets of mica, fuchsite, or hematite that produce a sparkling or glittering internal effect called aventurescence. Green aventurine (the most common, coloured by fuchsite mica inclusions) is widely used in commercial jewellery. Orange-red aventurine contains hematite platelets. Aventurine is frequently confused with jade simulants in the market; green aventurine (aventurine quartzite) is one of the materials most commonly sold as jade in tourist markets. A refractometer distinguishes aventurine quartz (RI approximately 1.55) from jadeite (RI approximately 1.66) immediately (GIA; Wise, 2016).
Price reference for quartz varieties (2024-25)
| Variety and quality | Approx. USD per carat |
|---|---|
| Chrysoprase, fine vivid green, eye-clean | USD 20-80 |
| Amethyst, deep Siberian colour, even, eye-clean, 10ct+ | USD 15-50 |
| Ametrine, well-defined bi-colour, natural Bolivian | USD 5-20 |
| Citrine, natural Madeira orange, eye-clean | USD 10-30 |
| Commercial amethyst, good purple | USD 1-10 |
| Commercial citrine (heat-treated amethyst) | USD 1-5 |
| Rose quartz, vivid pink, cabochon | USD 1-10 |
| Carnelian, fine orange, natural | USD 1-5 |
| Tiger's eye, fine chatoyancy | USD 1-8 |
| Commercial agate, onyx, aventurine | USD 0.50-5 |
Approximate ranges 2024-25. Quartz is the most affordable major gem family. Exception: exceptional collector amethyst and ametrine can exceed these ranges. Sources: GIA; Wise (2016); dealer benchmarks. Not price guarantees.
Frequently asked questions
Is dyed agate a fraud?
Dyed agate is a commercial standard, not inherently a fraud, when it is sold at dyed agate prices and the buyer knows they are buying dyed material. The problem occurs when dyed agate is sold as natural-colour material at natural-colour premiums, or when the dye is unstable and fades. Most commercial coloured agate (vivid blue, vivid green, vivid red) is dyed, and this is known and accepted in the trade at commercial prices. The concerns are: seller disclosure (some do not disclose dyeing), dye stability (some dyes fade with UV exposure or cleaning), and the risk of buying at natural premiums. For anything above a few dollars per piece, ask about dyeing and, for significant purchases, test with a GIA or qualified gemologist (GIA; Wise, 2016).
Can green aventurine be used as a jade substitute?
Aventurine quartzite (green aventurine) is widely sold as jade in tourist markets and by uninformed dealers. It is not jade by any definition: it is quartz, not jadeite or nephrite. It has different optical properties (notably the aventurescence sparkle absent in fine jade), different refractive index (detectable with a refractometer), and different value. It is a legitimate gem material in its own right at its own price level, but it is not jade and should not be priced or represented as jade. The comparison is important: fine jadeite may cost USD 50,000-1,000,000 per piece; green aventurine costs USD 1-5 per carat.
What is the difference between amethyst and purple sapphire?
Amethyst is silicon dioxide (SiO2, quartz), while purple sapphire is aluminium oxide (Al2O3, corundum). They are completely different minerals. Purple sapphire has Mohs hardness 9 versus amethyst's 7, is far rarer, and commands dramatically higher per-carat prices. The colour of fine purple sapphire (vivid violet-purple) can resemble the finest amethyst, but a refractometer immediately distinguishes them: amethyst RI approximately 1.54, sapphire RI approximately 1.76. At equivalent colour quality and size, purple sapphire costs 10-100 times more than amethyst. The minerals are unrelated despite colour similarity.
Sources cited in this article
- GIA Gem Reference Guide. (2006). Gemological Institute of America. (pp. 90-95)
- Wise, R.W. (2016). Secrets of the Gem Trade (2nd ed.). Brunswick House Press. (pp. 291-310)
- GIA Colored Stone identification. gia.edu.