Lazulite is a magnesium-iron-aluminium phosphate of formula (Mg,Fe)Al₂(PO₄)₂(OH)₂, occurring in shades of sky-blue through deep violet-blue, and prized by collectors as one of the rarer collector-grade blue gem species. It is not to be confused with lazurite, the principal blue mineral of lapis lazuli, although the names are routinely confused even in print; lazulite is a phosphate, lazurite is a feldspathoid silicate, and the two have nothing in common beyond a syllable and a colour.

The species was first described in 1795 from material at Krieglach in Styria, Austria, with the name derived (eventually) from the Persian lazaward, meaning blue. It crystallises in the monoclinic system, generally as steep dipyramidal crystals with a vitreous to dull lustre, and shows a Mohs hardness of 5.5-6 and a specific gravity of about 3.1. Its refractive indices fall in the 1.61-1.64 range, and it is biaxial negative with strong pleochroism: colourless or pale yellow on one axis, deep blue on the other. The pleochroism is one of its most distinctive identification clues for cut stones.

Gem-quality material is uncommon. The most important localities for facetable rough are the Yukon Territory of Canada (Rapid Creek and the Big Fish River area), Brazil (Minas Gerais, particularly the Itinga district), Madagascar, and historically Graves Mountain in Georgia, USA. Yukon material from the Crosscut Creek and Big Fish localities is especially prized for the saturation and tone of its blue, with the best stones approaching the depth of fine sapphire although with much lower brilliance owing to the lower refractive index. Crystals are generally small, and clean facetable material above one carat is genuinely scarce; stones above five carats are exceptional and command collector premiums well above what the species would otherwise warrant.

For the cutter, lazulite presents specific challenges. The strong cleavage on the (110) plane requires that the rough be oriented carefully on the dop, and the stone is somewhat heat-sensitive: rapid changes during dopping or polishing can craze the surface. The strong pleochroism rewards careful axis selection, with the c-axis oriented to bring out the deepest blue face-up. Cabochons of pegmatitic Brazilian and Madagascan material are also seen and can be very attractive, although they show less of the species' diagnostic optical character than a properly oriented faceted stone.

In a jewellery context, lazulite is a collector's stone rather than a commercial jewellery gem. Its hardness limits it to pieces that will not see daily wear (pendants, earrings, occasional rings worn with care), and its rarity limits supply to a small specialist circuit of dealers who handle one-of-a-kind cut stones. It is not heat-treated or otherwise enhanced in any commercial sense; the colour seen in a stone is the colour as it occurs in the rough.