Lapis Lazuli and Amethyst

Lapis Lazuli: Metamorphic Rock; Amethyst: Mineral (variety of Quartz)

Lapis Lazuli (a rock composed primarily of lazurite, calcite, and pyrite) and Amethyst (a variety of quartz, SiO2)

Also known as: Lapis Lazuli: Lapis; Amethyst: Purple Quartz

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Description

Lapis Lazuli is a deep-blue metamorphic rock prized since antiquity for its intense color. It is not a single mineral but a rock composed primarily of lazurite (a feldspathoid mineral), along with calcite (white streaks), and pyrite (golden flecks). Its characteristic blue color is due to the presence of the polysulfide anion (S3-) within the lazurite structure. Amethyst is a popular purple variety of quartz (SiO2), one of the most abundant minerals in the Earth's crust. Its color ranges from a light lilac to a deep, rich purple, sometimes with reddish or bluish secondary hues. The color is caused by trace amounts of iron impurities (Fe3+) within the crystal lattice, which are then subjected to natural irradiation.

How to Identify

Color: Lapis Lazuli: Intense azure to violet-blue, often with white calcite veins and golden pyrite specks. Amethyst: Shades of purple, from pale lilac to deep violet, sometimes with red or blue flashes.
Luster: Lapis Lazuli: Dull to greasy. Amethyst: Vitreous (glassy).
Texture: Lapis Lazuli: Fine-grained to granular, often with visible inclusions. Amethyst: Crystalline, typically forming prismatic crystals or drusy coatings.
Crystal Form: Lapis Lazuli: Massive, granular. Lazurite crystals within the rock are typically dodecahedral but rarely visible. Amethyst: Hexagonal prisms terminating in rhombohedral faces, often forming geodes or druzy crusts.
Cleavage: Lapis Lazuli: Poor to indistinct (due to its composite nature). Lazurite itself has indistinct cleavage. Amethyst: None (conchoidal fracture).
Geological Environment: Lapis Lazuli: Contact metamorphic zones, particularly in metamorphosed limestones or marbles. Amethyst: Hydrothermal veins, vugs, and geodes within igneous (especially basaltic) and metamorphic rocks.

Key Facts

Hardness: Lapis Lazuli: 5-5.5 (Mohs); Amethyst: 7 (Mohs)
Specific Gravity: Lapis Lazuli: 2.7-2.9; Amethyst: 2.65
Crystal System: Lapis Lazuli: Isometric (lazurite component); Amethyst: Trigonal
Color: Lapis Lazuli: Azure to violet-blue; Amethyst: Purple
Luster: Lapis Lazuli: Dull to greasy; Amethyst: Vitreous
Transparency: Lapis Lazuli: Opaque; Amethyst: Transparent to translucent
Fracture: Lapis Lazuli: Uneven to conchoidal; Amethyst: Conchoidal
Cleavage: Lapis Lazuli: Poor/Indistinct; Amethyst: None
Composition: Lapis Lazuli: Rock composed of lazurite (Na,Ca)8[(S,Cl,SO4,OH)2|(Al6Si6O24)], calcite (CaCO3), and pyrite (FeS2); Amethyst: Silicon Dioxide (SiO2)

Quick Check

Color: Lapis Lazuli: Deep blue with white and gold inclusions; Amethyst: Purple
Luster: Lapis Lazuli: Dull to greasy; Amethyst: Vitreous
Streak: Lapis Lazuli: Light blue (lazurite component), white (calcite component); Amethyst: White

Physical Characteristics

Crystal Habit: Lapis Lazuli: Massive, granular; Amethyst: Prismatic, often in geodes or drusy aggregates
Cleavage Type: Lapis Lazuli: Poor to indistinct (lazurite); Amethyst: None
Fracture Type: Lapis Lazuli: Uneven to conchoidal; Amethyst: Conchoidal
Tenacity: Lapis Lazuli: Brittle; Amethyst: Brittle
Luster Type: Lapis Lazuli: Dull to greasy; Amethyst: Vitreous

Formation

Lapis Lazuli: Formed through contact metamorphism of limestone or marble, where sulfur-rich fluids interact with calcium-rich rocks. The blue color is due to the lazurite component. Amethyst: Forms in hydrothermal solutions within igneous (especially volcanic) and metamorphic rocks. The purple color is due to irradiation of iron impurities (Fe3+) within the quartz crystal lattice.

Usage

Lapis Lazuli: Historically used for jewelry, ornamental objects, carvings, and as a pigment (ultramarine). Amethyst: Widely used as a gemstone for jewelry, ornamental carvings, and as a collector's mineral.

Age Distribution

Lapis Lazuli: Formed in various geological periods, often associated with regional metamorphism of carbonate rocks. Amethyst: Found in rocks of various ages, commonly in hydrothermal veins and volcanic geodes.

Where to Find

Badakhshan, Afghanistan (Lapis Lazuli)

The most historically significant and primary source of high-quality Lapis Lazuli, mined for over 6,000 years from the Sar-e-Sang mines.

Lake Baikal, Siberia, Russia (Lapis Lazuli)

Another important source, producing material often with a slightly lighter blue color and more prominent calcite.

Ovalle, Coquimbo Region, Chile (Lapis Lazuli)

Produces Lapis Lazuli, often with a paler blue color and more white calcite than Afghan material.

Minas Gerais, Brazil (Amethyst)

One of the world's largest and most important sources of amethyst, known for large geodes and high-quality crystals.

Artigas, Uruguay (Amethyst)

Famous for producing deep purple, often reddish-purple, amethyst geodes and crystals.

Zambia (Amethyst)

Known for producing amethyst with a rich, dark purple color, sometimes with a slight reddish flash.

Thunder Bay, Ontario, Canada (Amethyst)

Produces distinctive amethyst with red hematite inclusions, giving it a unique appearance.

Finding Tips

Lapis Lazuli Identification

Look for the characteristic deep blue color, often with white calcite streaks and golden pyrite flecks. A scratch test on an inconspicuous area might reveal a white streak (due to calcite), and a drop of dilute HCl will effervesce on the calcite portions.

Amethyst Identification

Identify by its purple color, vitreous luster, hexagonal crystal habit (if crystalline), and conchoidal fracture. Its hardness of 7 on the Mohs scale means it will scratch glass and steel.

Geological Context for Lapis Lazuli

Search in areas of contact metamorphism, particularly where limestones or marbles have been intruded by igneous rocks and subjected to sulfur-rich fluids.

Geological Context for Amethyst

Look for volcanic regions, especially in basalt flows where geodes can form, or in hydrothermal veins within various rock types.

Distinguishing Lapis Lazuli from Sodalite

Sodalite is typically a slightly duller blue, often with more prominent white veining and lacks the characteristic golden pyrite inclusions of Lapis Lazuli. Sodalite also has a white streak, while lazurite (the blue component of Lapis) has a blue streak.

Distinguishing Amethyst from Fluorite

Fluorite is much softer (Mohs 4) than amethyst (Mohs 7) and exhibits perfect octahedral cleavage, which amethyst lacks. Fluorite also has a wider range of colors, but purple fluorite can be confused with amethyst.

Similar Rocks

Sodalite

Na4Al3Si3O12Cl

Also known as: Blue Sodalite

Dumortierite

Al7(BO3)(SiO4)3O3

Also known as: Blue Quartz

Chalcedony

SiO2

Also known as: Cryptocrystalline Quartz

Fluorite

CaF2

Also known as: Fluorspar

Scientific Classification

Mineral Class: Lapis Lazuli: Rock (composed of silicates, carbonates, sulfides); Amethyst: Tectosilicate (Quartz Group)
Group: Lapis Lazuli: Not applicable (rock); Amethyst: Quartz Group
Crystal System: Lapis Lazuli: Isometric (lazurite); Amethyst: Trigonal
Chemical Formula: Lapis Lazuli: (Na,Ca)8[(S,Cl,SO4,OH)2|(Al6Si6O24)] (lazurite) + CaCO3 (calcite) + FeS2 (pyrite); Amethyst: SiO2
Composition: Lapis Lazuli: Sodium calcium aluminum silicate sulfate chloride hydroxide with calcium carbonate and iron sulfide; Amethyst: Silicon and Oxygen

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