Pyrite Inclusion in Lapis — The Golden Flecks That Define Fine Material
Pyrite Inclusion in Lapis — The Golden Flecks That Define Fine Material
Brassy pyrite crystals scattered through the deep blue lazurite matrix
Pyrite inclusions in lapis lazuli are brassy golden cubic or irregular grains of iron disulphide, FeS2, distributed through the deep blue lazurite-bearing matrix that gives lapis its characteristic colour. Far from being an undesirable feature, finely dispersed pyrite is considered an essential element of fine lapis: the visual contrast of warm gold flecks against intense ultramarine ground is one of the defining aesthetic features of the material and a marker that distinguishes natural lapis from its principal imitations.
Geological setting
Lapis lazuli is a metamorphic rock rather than a single mineral, composed principally of lazurite (the blue feldspathoid responsible for the colour) with subordinate calcite, sodalite, hauyne, and pyrite. The pyrite forms during the high-temperature metamorphism of pyritic limestones, where iron and sulphur from the protolith recrystallise into cubic pyrite alongside the lazurite. The classical Sar-e-Sang deposits in the Badakhshan Province of Afghanistan, worked since at least the third millennium BCE, produce lapis with the most prized combination of deep blue ground and finely dispersed golden pyrite. Other significant deposits at Lake Baikal in Russia and in Chile produce lapis with different pyrite distributions and ground colours.
Quality criteria
The finest lapis displays a uniform, deep ultramarine ground with evenly distributed small golden pyrite flecks and minimal white calcite. Excessive pyrite, particularly in coarse cubic clusters, can detract from the primary blue colour and is generally a quality concern; modest amounts of fine pyrite scattered evenly through the stone are considered desirable. Calcite presence is generally undesirable, with the highest grades showing little to no visible white veining. The Afghan trade traditionally distinguishes several grades, with the finest material — sometimes called nili — showing nearly pure deep blue with subtle pyrite and almost no calcite.
Diagnostic value and identification
Pyrite inclusions are diagnostic for natural lapis lazuli and help distinguish the material from its imitations. Imitation lapis — Gilson synthetic lapis, dyed jasper, dyed howlite, and various plastics — usually lacks pyrite or contains unconvincing imitation pyrite that fails the streak test. Natural pyrite gives the characteristic greenish-black to brownish-black streak; imitation pyrite typically shows no streak or an obvious paint or plastic appearance under magnification. See also lapis lazuli and synthetic lapis.