Spinel gemstones

thumb|upright=1.2|Magnetite is one of the very few minerals that is ferrimagnetic; it is attracted by a [[magnet as shown here]] thumb|Unit cell of magnetite. The gray spheres are oxygen, green are divalent iron, blue are trivalent iron. Also shown are an iron atom in an octahedral space (light blue) and another in a tetrahedral space (gray). Magnetite is a mineral and one of the main iron ores, with the chemical formula . It is one of the oxides of iron, and is ferrimagnetic; it is attracted to a magnet and can be magnetized to become a permanent magnet itself. With the exception of extremel

thumb|Polyhedral representation of spinel MgAl2O4 Spinel () is the magnesium/aluminium member of the larger spinel group of minerals. It has the formula in the cubic crystal system. Its name comes from the Latin word , a diminutive form of spine, in reference to its pointed crystals.

thumb|Main chrysoberyl producing countries

Chromite is a crystalline mineral composed primarily of iron(II) oxide and chromium(III) oxide compounds. It can be represented by the chemical formula of FeCr2O4. It is an oxide mineral belonging to the spinel group. The element magnesium can substitute for iron in variable amounts as it forms a solid solution with magnesiochromite (MgCr2O4). Substitution of the element aluminium can also occur, leading to hercynite (FeAl2O4). Chromite today is mined particularly to make stainless steel through the production of ferrochrome (FeCr), which is an iron-chromium alloy.

Galaxite, also known as 'mangan-spinel' is an isometric mineral belonging to the spinel group of oxides with the ideal chemical formula . It is sometimes used as a gemstone.

Ceylonite (first cited 1793) and pleonaste (first cited 1801) or pleonast are dingy blue or grey to black varieties of spinel. Ceylonite, named for the island of Ceylon, is a ferroan spinel with Mg:Fe from 3:1 and 1:1, and little or no ferric iron. Pleonaste is named from the Greek for 'abundant,' for its many crystal forms, and is distinguished chemically by low Mg:Fe ratios of approximately 1:3. It is sometimes used as a gemstone.