Is Phenakite a Beryl?
Phenakite is not a beryl: Be2(SiO4) against Be3Al2(Si6O18), uniaxial positive against negative, 16.37% beryllium against 5.03%. Where the balance fails.
No. Phenakite, Be2(SiO4), and beryl, Be3Al2(Si6O18), are separate species on the IMA list. Both are hexagonal beryllium silicates with hardness 7.5 to 8, which is the confusion. Optics settle it: phenakite is uniaxial positive, ω from 1.650; beryl uniaxial negative, no index above 1.610. Phenakite holds 16.37% beryllium, 3.25 times beryl’s 5.03%.
In short
- Two species. The September 2026 IMA-CNMNC Master List lists phenakite, Be2(SiO4), G, 1833, Russia, and beryl, Be3Al2(Si6O18), G, with year ? and country unknown. Phenakite has no aluminium and one silicon per formula; beryl’s formula brackets a six-silicon (Si6O18) group. Neither name is a variety of the other.
- Why they get mixed up. On their Handbook of Mineralogy sheets both are hexagonal, both have hardness 7.5 to 8, both run colourless, yellow and pink, and phenakite’s sheet lists beryl among its associates. Hardness and colour cannot separate them.
- The sign-and-index rule. Phenakite is uniaxial (+), ω 1.650 to 1.656, ε 1.667 to 1.670. Beryl is uniaxial (−), ω 1.567 to 1.610, ε 1.565 to 1.599. A refractometer reading of 1.650 or more is 0.040 clear of any beryl index on the sheet, and the optic sign is opposite. Either one decides it; both together leave no doubt.
- Documented failure: the balance. A 20.00 g piece reads 13.17 to 13.33 g in water as phenakite and 12.40 to 13.27 g as beryl. The 0.10 g from 13.17 to 13.27 g is shared, at the alkali-rich end of beryl, whose sheet says density increases with alkali content. Below 13.17 g it is not phenakite; above 13.27 g it is not beryl; between, weigh nothing into the answer.
- More beryllium, not a beryl. Worked on standard atomic weights, phenakite is 16.37% beryllium and beryl 5.03%: 3.25 times as much. The ‘is phenakite a quartz, a diamond or a tektite’ questions are answered below with the same sheets; the answer is no to each.
| Criterion | Phenakite | Beryl | Quartz | Diamond | Separates phenakite from beryl? |
|---|---|---|---|---|---|
| IMA formula | Be2(SiO4) | Be3Al2(Si6O18) | SiO2 | C | Yes |
| IMA status, year, country | G, 1833, Russia | G, ?, unknown | A, 1967 s.p., unknown | G, ?, unknown | — |
| Beryllium by mass (worked) | 16.37% | 5.03% | 0 | 0 | Yes: 3.25 times |
| Crystal system | Hexagonal (-3) | Hexagonal (6/m 2/m 2/m) | Hexagonal (32) | Cubic | No |
| Hardness | 7.5–8 | 7.5–8 | 7 | 10 | No |
| Density (measured) | 2.93–3.00 | 2.63–2.97 | 2.65 | 3.511 | Partly |
| 20.00 g piece in water | 13.17–13.33 g | 12.40–13.27 g | 12.45 g | 14.30 g | Fails from 13.17 to 13.27 g |
| Optical class | Uniaxial (+) | Uniaxial (−) | Uniaxial (+) | Isotropic | Yes |
| Refractive indices | ω 1.650–1.656, ε 1.667–1.670 | ω 1.567–1.610, ε 1.565–1.599 | ω 1.544, ε 1.553 | n 2.4175 (589 nm) | Yes: 0.040 gap |
| Birefringence (worked) | 0.011–0.020 | Not worked (ranges unpaired) | 0.009 | None | — |
| Cleavage | Distinct {11-20}; imperfect {10-11} | Imperfect {0001} | Rarely observable | {111} perfect | No |
| Colour (sheet) | Colourless, yellow, pink, brown | Colourless, blues, greens, yellow, rose, pink to red | Colourless, white and many others | Colourless, yellows and others | No |
Is phenakite a type of beryl? The IMA answer
Phenakite is not a type of beryl: it has its own row on the IMA list, a different formula and a different point group. The September 2026 list gives phenakite, Be2(SiO4), status G, 1833, Russia, and beryl, Be3Al2(Si6O18), status G, with the year printed as ? and the country as unknown. A variety name such as aquamarine has no row and shares beryl’s formula; the test is set out on is aquamarine a beryl. Phenakite fails it at the first step: no aluminium, and silicon as a single (SiO4) rather than beryl’s (Si6O18).
What they do share. The Handbook of Mineralogy phenakite sheet and the beryl sheet both say hexagonal, though phenakite’s point group is -3 and beryl’s 6/m 2/m 2/m; both give hardness 7.5 to 8; both include colourless, yellow and pink. Phenakite’s association line names topaz, beryl, chrysoberyl, fluorite, muscovite and quartz, and its occurrence is pegmatites, greisens and alpine and hydrothermal veins; beryl’s association line does not name phenakite. Both Distribution fields name Mt. Antero, Colorado. So a pegmatite specimen can carry both, and neither is a variety of the other.
The name. Phenakite’s sheet derives it from the Greek for deceiver, for its similarity to quartz, not to beryl; beryl’s gives the Greek beryllos, original significance unknown. Other beryl questions on this site are is euclase a beryl, is chrysoberyl a beryl and the species page beryl; their own tests are linked, not repeated.
How do you tell phenakite from beryl? The sign-and-index rule
The rule: uniaxial positive with an index of 1.650 or more is phenakite; uniaxial negative with no index above 1.610 is beryl. Phenakite’s sheet prints ω 1.650 to 1.656 and ε 1.667 to 1.670, so its extraordinary ray is the higher index, which is what positive means. Beryl’s prints ω 1.567 to 1.610 and ε 1.565 to 1.599, the extraordinary ray lower, negative. The closest pair, phenakite’s lowest ω against beryl’s highest ω, are 0.040 apart. The beryl sheet says its indices increase with alkali content, and even its top value stays below phenakite’s floor.
Who the rule is wrong for. It separates these two only. Quartz is uniaxial positive too, so a positive sign alone does not say phenakite; quartz’s indices, ω 1.544 and ε 1.553, are far below. Phenakite’s birefringence, worked from its printed ranges, is 0.011 to 0.020; quartz’s is 0.009. Euclase overlaps phenakite’s ω, and is biaxial; that separation is on the euclase page. The International Gem Society phenakite article gives an RI of 1.654 to 1.760, which does not match its own ε of 1.670; this page uses the Handbook figures.
Documented failure: the balance. A 20.00 g phenakite reads 13.17 to 13.33 g in water; a 20.00 g beryl 12.40 to 13.27 g. The shared 0.10 g belongs to the densest, alkali-rich beryl. A reading below 13.17 g rules phenakite out, and above 13.27 g rules beryl out, but most beryl sits lower, near the sheet’s 2.7 average, 12.59 g, so the balance helps only at the ends; the method is on how to measure specific gravity at home. Hardness does not help at all: both sheets print 7.5 to 8, and testing a cut stone damages it; see how to test mineral hardness properly.
Does phenakite have more beryllium than beryl? Worked
Yes: 16.37% beryllium by mass against 5.03%, or 3.25 times as much. On standard atomic weights (Be 9.0122, Al 26.9815, Si 28.085, O 15.999), Be2SiO4 weighs 110.11, of which two beryllium are 18.02: 16.37%, or 45.43% BeO. Be3Al2Si6O18 weighs 537.49, of which three beryllium are 27.04: 5.03%, or 13.96% BeO. Both BeO figures are the ideal columns printed on the two sheets.
Why. Per formula unit beryl has more beryllium atoms, three against two, but its six silicons, two aluminiums and eighteen oxygens make it nearly five times heavier. Phenakite’s formula has nothing but beryllium, silicon and oxygen. The USGS Professional Paper 1802-E, Beryllium (Foley and others, 2017) says bertrandite and phenakite are the predominant beryllium ore minerals in near-surface volcanogenic and carbonate-hosted deposits, and beryl the main one in plutonic and metamorphic systems.
Toxic? One of the questions people type is whether phenakite is toxic. That is a hazard question this page does not answer: beryllium is routed to toxic minerals in a collection, and nothing here is advice.
Is phenakite a quartz, a diamond or a tektite?
No to each. Quartz is SiO2 on the IMA list, with no beryllium; the quartz sheet gives density 2.65, so a 20.00 g piece reads 12.45 g in water against phenakite’s 13.17 to 13.33 g, and indices 1.544 and 1.553 against 1.650 and up. The full quartz comparison is on phenakite, and what makes quartz a mineral on is quartz a mineral. Diamond is carbon, cubic and isotropic on its sheet, hardness 10, density 3.511, n 2.4175; a 20.00 g diamond reads 14.30 g in water, 0.97 g above phenakite’s top. IGS records near-colourless phenakite rough submitted as suspected diamond, and says phenakite has lower dispersion, hardness and specific gravity, and that diamond has no birefringence. More on diamond is on is diamond a mineral.
Tektite. A whole-word script over the September 2026 IMA list finds no line containing tektite, so it is not a mineral species on that list; phenakite has its own row. What a name without an IMA row can still be is discussed on crystal vs mineral: what is the difference.
What to write on a label. The species name from the IMA list, phenakite or beryl, with any variety name after it. The species pages are phenakite and beryl; bertrandite is the low-temperature hydrated relative of phenakite. More classification questions are in the collecting guides. Nothing on this site is offered for sale; the wanted list is the only commercial route.