Lapis lazuli is a deep-blue metamorphic rock prized as a semi-precious stone since antiquity. Its signature color comes from the mineral lazurite, but the rock is a natural aggregate of several components. The stone often exhibits visible, sparkling metallic inclusions that resemble tiny flakes of gold. This appearance leads to common questions about the material’s actual composition and gives the stone a unique, starry look.
The Truth About the Golden Flecks
Lapis lazuli does not contain actual gold, despite the appearance of the bright, golden-colored specks embedded within the deep blue matrix. The brilliant inclusions are a completely different mineral.
The metallic-looking material is, in fact, the mineral pyrite, a naturally occurring iron sulfide. Pyrite’s pale brass-yellow hue and bright metallic luster make it look strikingly similar to gold to the untrained eye. This visual deception is so widespread that pyrite earned the enduring nickname “Fool’s Gold.”
Understanding Pyrite’s Composition
Pyrite is chemically identified as iron disulfide (FeS2). It is the most abundant sulfide mineral found across various rock types. The brassy color and bright luster come from its crystal structure and metallic bonds.
Pyrite forms as an accessory mineral within the lapis lazuli rock, which is an aggregate primarily composed of lazurite, calcite, and pyrite. Lapis lazuli forms through the metamorphism of limestone, a geological process where these minerals crystallize together. The deep blue of the stone is due to the lazurite component, while the pyrite and white calcite are naturally incorporated during the rock’s formation. The pyrite is present as small, discrete grains scattered throughout the stone.
Pyrite’s Impact on Lapis Lazuli Quality
The presence and distribution of pyrite have a direct effect on the commercial value and aesthetic appeal of lapis lazuli. Stones with small, finely-dispersed golden flecks are highly valued by consumers and gemologists. These inclusions add a desirable contrast and sometimes acting as a marker of the stone’s authenticity.
Conversely, the quality of the stone decreases if the pyrite is present in large, unattractive patches or is overly concentrated. An excess of pyrite can cause a dull, greenish tint, which detracts from the prized deep blue color. Similarly, large veins or streaks of white calcite, another mineral present in the rock, will also lower the stone’s overall desirability. The highest quality lapis lazuli is characterized by an intense, uniform, deep blue color with only a light, attractive sprinkling of pyrite.