Is Prehnite a Quartz?
Is prehnite a quartz? No: Ca2Al(Si3Al)O10(OH)2, hardness 6–6.5, density 2.80–2.95 against quartz’s SiO2, 7 and 2.65. Its index sits 0.058 higher. Jade too.
No. Prehnite is Ca2Al(Si3Al)O10(OH)2 on the September 2026 IMA list, a calcium aluminium silicate: orthorhombic, hardness 6–6.5, density 2.80–2.95. Quartz is SiO2: hexagonal, hardness 7, density 2.65. Prehnite is not chalcedony either, and its lowest refractive index, 1.611, sits 0.058 above quartz’s highest. GIA’s jade is jadeite or nephrite, not prehnite.
In short
- Two species. The September 2026 IMA-CNMNC Master List has a row for prehnite, Ca2Al(Si3Al)O10(OH)2 (G, 1788, South Africa), and a row for quartz, SiO2 (A, 1967 s.p.). Prehnite is 43.71% SiO2 by mass when worked as oxides, and that silica is bound with calcium, aluminium and hydroxyl in a different structure: orthorhombic on its Handbook sheet, hexagonal for quartz. Not a quartz, and, as is prehnite a zeolite shows, not a zeolite.
- The 0.058 gap. The prehnite sheet prints α 1.611–1.632, β 1.615–1.642, γ 1.632–1.665; the quartz sheet ω 1.544, ε 1.553. No prehnite index comes within 0.058 of quartz’s, and the International Gem Society chalcedony page gives chalcedony 1.540–1.553, the same ceiling. On a refractometer prehnite reads about 1.63 (IGS and Hobbs (1982)), quartz and chalcedony below 1.56.
- The blink. Worked from the sheet, prehnite’s birefringence is 0.021 at the low ends of its ranges and 0.033 at the high ends; quartz’s is 0.009. Hobbs, The Jade Enigma, Gems & Gemology (1982) describes turning a polaroid plate at the refractometer eyepiece: prehnite shows a slight but noticeable shadow movement, the “birefringence blink”, of 0.020–0.033.
- Documented failure: nephrite. On Hobbs’s property chart nephrite reads 1.606–1.632 against prehnite’s 1.616–1.649, and 2.95 ± 0.05 against 2.88 ± 0.06. Hobbs writes that only prehnite could be confused with nephrite on specific gravity, and that the refractometer makes the distinction. In water, 20.00 g reads 12.86–13.22 g as prehnite (sheet) and 13.10–13.38 g as nephrite (IGS): the ranges overlap.
- Hardness and weight. Prehnite is 6–6.5 against quartz’s 7, so quartz scratches prehnite, but only just (how to test mineral hardness properly). Prehnite is 1.06 to 1.11 times as dense: 20.00 g reads 12.86–13.22 g in water against quartz’s 12.45 g, a gap of at least 0.41 g (how to measure specific gravity at home).
| Material | What it is (source) | System | Hardness | Density | 20.00 g in water | Refractive index | Spot reading (Hobbs) | Birefringence |
|---|---|---|---|---|---|---|---|---|
| Prehnite | Ca2Al(Si3Al)O10(OH)2; IMA species | Orthorhombic | 6–6.5 | 2.80–2.95 | 12.86–13.22 g | α 1.611–1.632 … γ 1.632–1.665 | 1.63 | 0.021–0.033 (blink 0.020–0.033) |
| Quartz | SiO2; IMA species | Hexagonal | 7 | 2.65 | 12.45 g | ω 1.544, ε 1.553 | 1.54, 1.55 | 0.009 |
| Chalcedony | Quartz in microscopic crystals; no IMA row (IGS) | Not stated | 7 | 2.55–2.91 (IGS) | 12.16–13.13 g | 1.540–1.553 (IGS) | 1.53 | 0.004–0.009 (IGS) |
| Jadeite | NaAlSi2O6; IMA species, one jade mineral (GIA) | Monoclinic | 6–7 | 3.24–3.43 | 13.83–14.17 g | α 1.640–1.681 … γ 1.652–1.692 | 1.66 | Rarely shows (Hobbs) |
| Nephrite | Tremolite–actinolite amphibole rock; jade (GIA); no IMA row | Not stated | 6–6.5 (IGS) | 2.90–3.02 (IGS) | 13.10–13.38 g | 1.600–1.641 (IGS) | 1.61–1.62 | Rarely shows (Hobbs) |
Is prehnite a type of quartz? Two different species
No: prehnite and quartz have separate rows on the IMA list and different formulas, structures and numbers. The September 2026 IMA-CNMNC Master List writes prehnite Ca2Al(Si3Al)O10(OH)2, status G, 1788, South Africa, and quartz SiO2, status A, 1967 s.p.; what makes quartz a mineral species is on is quartz a mineral. The prehnite sheet heads it Ca2Al2Si3O10(OH)2, the same atoms, with an ideal composition of SiO2 43.71, Al2O3 24.72, CaO 27.20 and H2O 4.37; worked from the IMA formula on standard atomic weights the figures are 43.71, 24.73, 27.20 and 4.37.
Where the confusion comes from. Prehnite is a silicate, and its sheet’s colours, light to dark green, white, yellow, gray and pink, overlap the colours quartz takes from inclusions. Its aggregates are fanlike, reniform, globular or stalactitic, with a vitreous, weakly pearly lustre, so a polished green piece can pass for green quartz, chalcedony or jade. The quartz sheet prints hexagonal symmetry, hardness 7, density 2.65 and uniaxial (+) optics; the prehnite sheet prints orthorhombic, 6–6.5, 2.80–2.95 and biaxial (+).
Not a zeolite either. Prehnite grows with zeolites; the IMA structural test that excludes it is on is prehnite a zeolite and is not repeated here. The prehnite species page carries the rest of its data.
How do you tell prehnite from quartz? The 0.058 gap
On a refractometer the two do not come close: prehnite’s lowest printed index, 1.611, is 0.058 above quartz’s highest, 1.553. The prehnite sheet gives α 1.611–1.632, β 1.615–1.642 and γ 1.632–1.665; the International Gem Society prehnite page says a refractometer usually gives a shadow edge about 1.63, and Hobbs, The Jade Enigma, Gems & Gemology (1982) gives a spot reading of 1.63. Quartz’s sheet gives 1.544 and 1.553, and Hobbs’s spot readings for quartz are 1.54 and 1.55.
The blink confirms it. Quartz’s birefringence is 0.009. Prehnite’s, worked from the sheet at matched range ends, is 0.021–0.033, which is the 0.021–0.033 the IGS page prints and the 0.020–0.033 “birefringence blink” Hobbs describes: with a polaroid plate rotated at the eyepiece, prehnite’s shadow edge moves slightly but noticeably inside the spot. Prehnite is also biaxial and quartz uniaxial.
Without a refractometer. Quartz, 7, scratches prehnite, 6–6.5, but the margin is half a step, so a scratch is a weak test (how to test mineral hardness properly). The balance is better: a 20.00 g piece reads 12.45 g in water as quartz and 12.86–13.22 g as prehnite, at least 0.41 g apart (how to measure specific gravity at home). Green aventurine is a separate question, answered on is aventurine a quartz.
Is prehnite chalcedony? No, and the numbers say so
Chalcedony is quartz in crystals too small to see; prehnite is a different mineral, even when it is massive and translucent. The quartz sheet uses “chalcedony” for its fine-grained to microcrystalline habit, and a script over the IMA list finds no row for chalcedony, as is chalcedony quartz also found. The International Gem Society chalcedony page gives chalcedony hardness 7, refractive index 1.540–1.553, birefringence 0.004–0.009 and specific gravity 2.55–2.91; Hobbs’s chart gives 1.535–1.539, spot 1.53, and 2.60 ± 0.05.
Documented failure: density. IGS’s chalcedony range, 2.55–2.91, reaches into prehnite’s 2.80–2.95, so 20.00 g reads 12.16–13.13 g as chalcedony and 12.86–13.22 g as prehnite. A reading between 12.86 and 13.13 g decides nothing. The refractometer does: 1.53–1.55 for chalcedony against about 1.63 for prehnite.
On the label. A green, waxy, crystal-free piece that reads near 1.63 is not chalcedony and should not be catalogued as quartz. The chalcedony page has chalcedony’s own composition.
Is prehnite jade? Not in gemmology, but it has been sold as one
No. GIA defines jade as a generic term for nephrite, jadeite and, under certain conditions, green omphacite; prehnite is none of them. Hobbs, The Jade Enigma, Gems & Gemology (1982) lists “Japanese jade” for prehnite in its table of misleading terms for jade simulants; the naming question as a whole is on is jade a mineral.
Documented failure: prehnite and nephrite overlap. On Hobbs’s property chart nephrite reads 1.606–1.632, spot 1.61–1.62, and 2.95 ± 0.05; prehnite reads 1.616–1.649, spot 1.63, and 2.88 ± 0.06. Hobbs writes that, of ten jade simulants, only prehnite could be confused with nephrite on specific gravity, in which case the refractometer would make the distinction; Hobbs’s tool for prehnite is the birefringence blink, because nephrite and jadeite rarely show any. jadeite is easier: its sheet gives 3.24–3.43 and indices from 1.640, and a 20.00 g piece reads 13.83–14.17 g in water.
What this page does not do. It does not judge a piece sold as jade; the scale and refractometer method for that is on how to tell if jade is real, and UV behaviour is on is prehnite UV reactive. More identification pages are in the collecting guides. Nothing on this site is offered for sale; the wanted list is the only commercial route.