Spinel
Spinel, MgAl2O4, is cubic, hardness 7.5 to 8, density 3.6 to 4.1, one index 1.719. How it differs from ruby and pyrope, and how the spinel series is named.
Spinel is magnesium aluminium oxide, MgAl2O4: cubic, hardness 7.5 to 8, density 3.6 to 4.1, a single refractive index of 1.719, typically octahedral. It is red, pink, blue and many other colours. GIA says the Black Prince’s Ruby in the Imperial State Crown is a red spinel; ruby is corundum, which gives two refractometer readings.
In short
- One species in a supergroup. The September 2026 IMA-CNMNC Master List lists spinel as MgAl2O4, grandfathered. The Handbook of Mineralogy sheet places it in the spinel supergroup, oxyspinel group, spinel subgroup, in series with magnesiochromite, gahnite and hercynite. Bosi, Biagioni and Pasero (2019), abstract name each species by the dominant A and B cations in the formula AB2X4.
- The Kaveltorp check. The sheet’s Kaveltorp, Sweden analysis is 16.47% ZnO, 37.1% of pure gahnite’s 44.39%, worked. Its A site still reads Mg 0.57 against Zn 0.33, so by the dominant-cation rule the material is spinel. Density rises with Fe and Zn, which is why the sheet’s measured range runs from 3.6 to 4.1.
- Red spinel is not ruby. Spinel is cubic and gives one refractometer reading, 1.719; ruby is corundum and gives two, 1.759 to 1.772 on its sheet. At GIA’s specific gravities, 3.60 and 4.00, a 20.00 g piece reads 14.44 g in water as spinel and 15.00 g as ruby, 0.56 g apart. GIA says chromium reddens both.
- The 0.03-gram trap. Pyrope garnet reads 14.42 g in water for 20.00 g, between spinel’s 14.41 g (calculated density) and 14.44 g (GIA). The sheets’ indices are 1.714 and 1.719, 0.005 apart, and the International Gem Society’s red spinel (1.715 to 1.735) and pyrope (1.73 to 1.766) ranges overlap. Crystal form decides: octahedra against dodecahedra.
- The names cut both ways. Spinels were sold as balas rubies (GIA; the International Gem Society). In the other direction, Grew et al., American Mineralogist 98 (2013) list 5 names containing spinel in their garnet name list, and all 5 are garnets: Arizona spinel, Arizona-spinell, kandy spinel and kandyspinell decode to almandine, Ceylonspinell to garnet.
| Mineral, IMA formula | Crystal system | Refractometer readings on the sheet | Hardness | Density | 20.00 g piece in water | Colours on the sheet (first listed) |
|---|---|---|---|---|---|---|
| Spinel, MgAl2O4 | Cubic | One: n = 1.719 | 7.5 to 8 | 3.6 to 4.1; 3.578 calculated | 14.41 to 15.12 g | Colourless, brown, black; red, orange, yellow, green, blue, indigo, violet |
| Corundum (ruby), Al2O3 | Hexagonal | Two: ω 1.767 to 1.772, ε 1.759 to 1.763 | 9 | 3.98 to 4.10 | 14.97 to 15.12 g | Pink to pigeon-blood-red among others |
| Pyrope (garnet), Mg3Al2(SiO4)3 | Cubic | One: n = 1.714 | 7 to 7.5 | 3.582 | 14.42 g | Purple-red, pinkish red, red-orange, deep red |
| Almandine (garnet), Fe2+3Al2(SiO4)3 | Cubic | One: n = 1.830 | 7 to 7.5 | 4.318 | 15.37 g | Deep red, brownish red, red-violet |
| Gahnite, ZnAl2O4 | Cubic | One: n = 1.79 to 1.80 | 7.5 to 8 | 4.38 to 4.60 | 15.43 to 15.65 g | Dark green, bluish green, blue to indigo, yellow to brown |
| Hercynite, Fe2+Al2O4 | Cubic | One: n = 1.80 to 1.83 | 7.5 to 8 | 4.40 | 15.45 g | Dark green to black |
| Magnesiochromite, MgCr2O4 | Cubic | One: n = 1.96 | 5.5 to 7 | 4.39 to 4.67 | 15.44 to 15.72 g | Black, deep red; metallic lustre |
What is spinel? Magnesium aluminium oxide, cubic
The names. The September 2026 IMA-CNMNC Master List gives spinel as MgAl2O4, status G, with the year printed as 1546 followed by a question mark and the country as unknown. The Handbook sheet writes the same formula and derives the name, perhaps, from the Latin spinella, little thorn, for the spine-shaped octahedral crystals. On standard atomic weights MgAl2O4 has a formula mass of 142.26 and is 28.33% MgO and 71.67% Al2O3, the sheet’s ideal column to the hundredth.
What it looks like, per the sheet. Cubic, point group 4/m -3 2/m, commonly as octahedra to 30 cm, which may show dodecahedron or cube faces; also coarse granular, rounded and massive. Twinning is on {111} as both twin and composition plane, the spinel law, by penetration or contact, and may be repeated as sixlings. GIA’s spinel description adds that twinned crystals look flattened, because the two pyramids of the octahedron rotate against each other during growth. The vocabulary is on mineral twinning explained.
Properties. Hardness 7.5 to 8; measured density 3.6 to 4.1, increasing with iron and zinc content, and 3.578 calculated; parting on {111}, no cleavage line; fracture conchoidal, uneven to splintery; brittle. Isotropic, n = 1.719; streak white; lustre vitreous, splendent to dull. The sheet prints no luminescence line, by a ligature-aware search in which hemimorphite and willemite are the positive controls.
Spinel, gahnite, hercynite: how the series is named
The Kaveltorp check: a zinc-rich spinel is still spinel while magnesium is the largest cation on the A site. Bosi, Biagioni and Pasero (2019), abstract say a spinel-supergroup species is identified by the combination of the dominant A and B cations in AB2X4. The spinel sheet’s Kaveltorp, Sweden analysis reads 16.47% ZnO and 14.08% MgO, with the formula (Mg0.57Zn0.33Fe2+0.07Mn2+0.02Ca0.01)(Al1.84Fe3+0.14Si0.02)O4. Worked, that ZnO is 37.1% of pure gahnite’s 44.39%; but magnesium, 0.57, outweighs zinc, 0.33, and aluminium is 92% of the B site. Its other analysis, labelled South East Asia, is Mg1.00 on the A site.
The series partners, on their sheets. Gahnite, ZnAl2O4, is the zinc end: density 4.38 to 4.60, n 1.79 to 1.80. Hercynite, Fe2+Al2O4, is the ferrous iron end: density 4.40, n 1.80 to 1.83, dark green to black. Magnesiochromite, MgCr2O4, swaps aluminium for chromium: density 4.39 to 4.67, n 1.96, metallic lustre, and its sheet records it as weakly magnetic (see which minerals are magnetic). Each heavier partner moves density and index up, which is the sheet’s note that density increases with Fe and Zn. Magnetite is in the same group on its sheet. The International Gem Society calls the Mg–Zn intermediate gahnospinel; the IMA list has no gahnospinel row, by script.
A formula shape is not membership. Bosi et al. set two criteria: a cation-to-anion ratio of 3:4, and a particular framework structure. By script, 43 rows of the IMA list have a formula written as one cation, two of a second and O4; among them spinel, gahnite, hercynite, magnesiochromite, chromite, magnetite, galaxite and franklinite, and also chrysoberyl, BeAl2O4, which its Handbook sheet makes orthorhombic. The abstract counts fifty-six supergroup minerals, 33 of them in the oxyspinel group; the formula shape alone cannot say which they are.
Is spinel a ruby? Red spinel against corundum
Red spinel is not ruby: spinel is MgAl2O4 and cubic, ruby is corundum, Al2O3, and hexagonal. GIA’s spinel history page says large spinels from central and southeast Asia became known as balas rubies, and that the Black Prince’s Ruby, set in England’s Imperial State Crown, is a crimson-red spinel, probably mined in the mountains of Afghanistan, which Edward, Prince of Wales received in 1367. The Timur Ruby, GIA adds, is another spinel, over 350 carats. GIA’s spinel page credits the mineralogist Jean Baptiste Louis Rome de Lisle, as GIA spells him, with identifying spinel as a different mineral from ruby, in 1783.
How the two separate. On the refractometer, spinel gives one reading and corundum two, the corundum page’s rule, linked rather than repeated: corundum. Spinel’s 1.719 sits 0.040 below corundum’s lowest index, 1.759. On the scale, at GIA’s specific gravities, 3.60 for spinel and 4.00 for ruby, a 20.00 g piece reads 14.44 g and 15.00 g in water, 0.56 g apart; the method is on how to measure specific gravity at home. Hardness, 7.5 to 8 against 9, needs a scratch and is for rough only: how to test mineral hardness properly.
Where the scale fails, documented. The spinel sheet’s measured density runs to 4.1, rising with iron and zinc, which takes it into corundum’s 3.98 to 4.10: an iron- or zinc-rich spinel can read 15.12 g, the same as corundum’s heaviest. Colour does not separate them either: GIA says red spinel is coloured by chromium, the trace element that colours ruby. The International Gem Society says balas ruby is acceptable only in discussions of history, and that terms such as ruby spinel wrongly imply a type of ruby.
Spinel or garnet? The 0.03-gram trap
The 0.03-gram trap: a 20.00 g pyrope reads 14.42 g in water and a 20.00 g spinel 14.41 to 14.44 g, so the scale cannot tell red spinel from pyrope garnet. Worked as 20 − 20/D from pyrope’s measured 3.582, spinel’s calculated 3.578 and GIA’s spinel figure of 3.60. Both are cubic and give one refractometer reading, 1.714 and 1.719 on their sheets, 0.005 apart. On the International Gem Society’s gem figures the ranges overlap: red spinel 1.715 to 1.735, pyrope 1.73 to 1.766.
What does separate them. Crystal form, on a crystal: the spinel sheet records octahedra, the pyrope sheet dodecahedra or trapezohedra. Almandine, the other red garnet, reads 15.37 g in water and n = 1.830, well clear of spinel. The live how to tell if garnet is real page uses spinel as the documented failure of its own refractometer line; that page’s method is not repeated here. The garnet itself is on pyrope.
Garnets called spinel. The confusion runs in both directions. In Appendix 2 of Grew et al., American Mineralogist 98 (2013), a list of 715 synonyms, obsolete, varietal and discredited names for garnet-supergroup minerals, a script finds 5 names containing spinel, and all 5 are garnets: Arizona spinel and Arizona-spinell (almandine), kandy spinel and kandyspinell (almandine), and Ceylonspinell (garnet). The International Gem Society’s list of misleading names, taken from Schumann, gives Arizona spinel as red or green garnet and candy spinel as almandite. The authors call their list necessarily incomplete.
Where spinel is found: Mogok, Sri Lanka and Franklin
The Occurrence field: a common mineral, formed at high temperatures as an accessory in igneous rocks, principally basalts, kimberlites and peridotites, and in xenoliths; in aluminium-rich schists; in regionally and contact metamorphosed limestones; detrital. Its associates are forsterite, chondrodite, scapolite, phlogopite, corundum, sillimanite and andalusite.
The Distribution field opens with many localities and names, for good specimens, gem crystals from around Mogok, Myanmar, and the Ratnapura and Elahera districts, Sri Lanka; many places from Amity and Edenville, New York to Andover, New Jersey; fine crystals from Franklin and Sterling Hill, New Jersey; near Midland and the Crestmore quarry, California; Burgess, Ontario and Wakefield, Quebec; the Eifel and Bodenmais, Germany; Monte Somma and Vesuvius, Italy; Andrahomana, Ambinda and Besakoa, Madagascar; large crystals from the Emeldjak and Katalakhu deposits, Aldan Shield, Russia; and Gorondarinskoye, Pamir Mountains, Tajikistan. No British locality is on the spinel sheet; the hercynite sheet names the Island of Mull, Scotland, for hercynite, the iron end of the series.
Synthetic spinel is common. GIA says its widespread use as an imitation of other gems has confused spinel’s identity; the International Gem Society says spinel has been synthesized since the 19th century, by flame fusion, flux and Czochralski methods, and that most colourless spinels in jewellery are likely synthetic. Synthetic blue spinel as an aquamarine imitation is on how to tell if aquamarine is real. The rest of the reference section is at the species index. We hold no catalogue and nothing here is offered for sale; the wanted list is the only commercial route.