Lazurite is the principal blue colour-bearing mineral in lapis lazuli. It is a feldspathoid silicate of the sodalite group, with the formula (Na,Ca)₈(AlSiO₄)₆(SO₄,S,Cl)₂, and it owes its intense blue colour to the trisulphur radical anion S₃^- trapped within the aluminosilicate framework. It is decisively distinct from lazulite, a magnesium-iron-aluminium phosphate; the two are persistently confused in print, and the easiest way to keep them apart is that lazurite makes up lapis, lazulite does not.

Lapis lazuli is a metamorphic rock rather than a single-mineral gem. Lazurite typically constitutes 20 to 40 percent of the rock by volume, with the remainder being calcite (the white veining and patches), pyrite (the gold flecks that connoisseurs consider the signature of fine Afghan material), and lesser amounts of diopside, hauyne, sodalite and various accessory minerals. The proportion and distribution of these phases determines the grade of the lapis: the very best material, sometimes called "Persian" or "Afghan first quality" in the trade, is a deep saturated violet-blue with no visible calcite and only a fine even sprinkling of pyrite.

The classical source of lazurite-bearing lapis is the Sar-i-Sang mining district of Badakhshan in north-eastern Afghanistan, which has been worked continuously for at least 7,000 years. Lapis from this region was already being traded across the ancient Near East by the fourth millennium BCE, and pieces of Afghan lapis are documented in the royal tombs of Ur, in pharaonic Egyptian jewellery and inlay, and in the gold mask of Tutankhamun. Other historical and modern sources include the Lake Baikal region of Russia (Slyudyanka), the Andean Cordillera in Chile (Coquimbo and Ovalle), and a small deposit at Italian Mountain in Colorado, USA. Of these, only Afghan and Chilean material is commercially significant today, with Afghan material commanding a premium of two to five times the Chilean equivalent for similar visible quality, owing to the depth and stability of its blue.

Gemmologically, lazurite as a single-mineral inclusion is rare on the cut-stone market; what reaches the trade is invariably the rock lapis lazuli. Lapis is normally fashioned as cabochons, beads, carvings, intaglios and inlay; it is too soft (Mohs 5-5.5) and porous for daily-wear ring use, and good lapis is almost always sealed with wax or paraffin to stabilise it against the absorption of skin oils and cosmetics. More aggressive treatments are seen in the lower commercial grades: dyeing with Prussian blue or with synthetic dyes is widespread on lower-quality Chilean material to mimic the depth of Afghan goods, and a careful dealer will check any deeply coloured cabochon with a cotton swab moistened with acetone, which will pull dye out of treated stones but not out of natural ones.

From a cultural standpoint, lazurite (as lapis) is the source of the historical pigment ultramarine, made by grinding and washing the rock in a process described by Cennino Cennini in the early fifteenth century. Until the synthesis of artificial ultramarine in 1828, natural lazurite-derived ultramarine was the most expensive pigment available to European painters, more expensive than gold leaf, and was reserved for the most important parts of an altarpiece (most famously, the robes of the Virgin Mary). That history is a useful reminder that the gem trade and the pigment trade in lazurite were, until quite recently, the same trade.