Amorphous Gemstones: Opal, Obsidian and Non-Crystalline Stones

Not all gemstones are crystals. A fascinating group of gems lacks the ordered atomic structure that defines crystalline minerals, yet they are among the most beautiful and prized stones in the world. Opal, obsidian, amber, and jet are all amorphous gems, and understanding what makes them different from crystalline stones reveals a whole new dimension of gem science.

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What Are Amorphous Gemstones?

The word amorphous comes from the Greek for without form. In mineralogy, an amorphous material is one that lacks long-range atomic order. Unlike crystalline gems where atoms are arranged in a precise, repeating three-dimensional pattern, amorphous gems have atoms arranged in a random or semi-random way, more like a frozen liquid than a true solid crystal.

Because they have no crystal structure, amorphous gems:

• Belong to no crystal system - they cannot be classified as cubic, trigonal, etc.
• Are optically isotropic - like cubic gems, they are singly refractive
• Show no cleavage - they break with conchoidal (shell-like) fracture instead
• Have no pleochroism - color appears the same from all directions

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Opal: The Amorphous Gem of Color

Opal is the most scientifically fascinating of all amorphous gems. Chemically, it is hydrated silicon dioxide (SiO2 plus water), but unlike quartz (which is also silicon dioxide), opal has no crystal structure. Instead, it consists of tiny spheres of silica arranged in a semi-ordered pattern.

How Opal's Play of Color Works

Precious opal owes its spectacular play of color to the arrangement of its silica spheres. When the spheres are uniform in size and arranged in a regular grid, they act as a diffraction grating for visible light. Different wavelengths of light are diffracted at different angles, producing flashes of spectral color as the viewing angle changes.

• Larger spheres (around 300 nanometers) diffract red light
• Medium spheres (around 250 nanometers) diffract green light
• Smaller spheres (around 200 nanometers) diffract blue and violet light

Common opal, which shows no play of color, has silica spheres that are irregular in size or arrangement, preventing coherent diffraction.

Opal Varieties

Variety	Description	Origin
White opal	Light body color with play of color	South Australia (Coober Pedy)
Black opal	Dark body color; most valuable	Lightning Ridge, New South Wales
Boulder opal	Thin opal layer on ironstone matrix	Queensland, Australia
Crystal opal	Transparent to translucent body	South Australia
Fire opal	Orange to red body color; may lack play of color	Mexico
Ethiopian opal	Hydrophane opal; absorbs water	Welo, Ethiopia

Opal Properties

• Hardness: Mohs 5.5 to 6.5
• Water content: 3 to 21 percent; most gem opals contain 6 to 10 percent water
• Refractive index: 1.37 to 1.47 (low, due to water content and amorphous structure)
• Stability: Can crack or craze if dehydrated; avoid heat and dry conditions

Opal Care

Because opal contains water in its structure, it requires special care. Avoid prolonged exposure to heat, dry air, or direct sunlight, which can cause dehydration and crazing (surface cracking). Store opal with a damp cloth in a sealed bag if kept in dry environments. Never use ultrasonic or steam cleaners on opal.

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Obsidian: Volcanic Amorphous Glass

Obsidian is natural volcanic glass formed when silica-rich lava cools so rapidly that atoms have no time to arrange into a crystal structure. It is one of the purest examples of an amorphous solid, with a composition similar to granite but a completely disordered atomic structure.

Obsidian Properties

• Hardness: Mohs 5 to 5.5
• Fracture: Perfect conchoidal fracture; produces extremely sharp edges
• Color: Usually black; also brown, gray, and rarely red or green
• Luster: Vitreous (glassy)

Obsidian Varieties

• Rainbow obsidian: Shows iridescent color bands caused by layers of tiny magnetite crystals
• Snowflake obsidian: Black with white cristobalite (a crystalline silica) inclusions
• Mahogany obsidian: Black with brown patches of iron oxide
• Apache tears: Small rounded nodules of obsidian, often translucent

Obsidian's conchoidal fracture produces edges sharper than surgical steel. Ancient peoples used it for cutting tools, arrowheads, and blades. Today, some surgeons use obsidian scalpels for procedures requiring extremely fine incisions.

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Amber: Fossilized Amorphous Resin

Amber is fossilized tree resin, typically 30 to 90 million years old. It is an organic amorphous material with no crystal structure. Amber is prized for its warm golden color, its inclusions of ancient insects and plant material, and its historical and cultural significance.

• Hardness: Mohs 2 to 2.5; very soft
• Refractive index: 1.539 to 1.545
• Fluorescence: Often shows blue to green fluorescence under UV light
• Inclusions: Insects, plant material, air bubbles, and water droplets preserved for millions of years
• Major sources: Baltic region (oldest and most abundant), Dominican Republic, Myanmar (Burmese amber)

Amber Identification and Imitations

Amber is frequently imitated by copal (young resin, less than 1 million years old), glass, and plastics. Key tests include the salt water float test (amber floats in saturated salt water; most plastics sink), hot needle test (amber smells like pine resin; plastics smell acrid), and UV fluorescence.

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Jet: Amorphous Organic Gem

Jet is a variety of lignite (brown coal) formed from ancient wood compressed and altered over millions of years. Like amber, it is an organic amorphous material. Jet has been used in jewelry since the Bronze Age and was particularly fashionable in Victorian mourning jewelry.

• Hardness: Mohs 2.5 to 4
• Color: Velvety black
• Luster: Waxy to velvety
• Major source: Whitby, England (Whitby jet is the finest quality)

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Other Amorphous and Partially Amorphous Gems

• Glass (natural tektites): Formed by meteorite impacts melting rock; includes moldavite and Libyan desert glass
• Metamict minerals: Crystals whose structure has been destroyed by radioactive decay; some zircon and euxenite
• Coral: Organic material; partially crystalline (aragonite) but often treated as amorphous in gemology
• Pearl: Composed of aragonite crystals but behaves optically like an amorphous material

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Identifying Amorphous Gemstones

• Polariscope: Amorphous gems appear isotropic (like cubic gems) - dark in all positions
• Refractometer: Single RI reading; opal's low RI is distinctive
• Observation: Conchoidal fracture, lack of cleavage, and glassy luster are characteristic
• UV light: Amber's fluorescence and opal's response help identification

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Frequently Asked Questions

Is opal a mineral?

This is debated. Traditionally, minerals must be crystalline, which would exclude opal. However, the International Mineralogical Association now accepts mineraloids (amorphous natural inorganic substances) as minerals, so opal is officially classified as a mineral despite lacking crystal structure.

Why does opal crack?

Opal contains water in its amorphous silica structure. When opal dries out, the silica spheres contract unevenly, creating internal stress that causes crazing (surface cracking) or fracturing. This is why opal should be kept away from heat and dry conditions.

How can I tell real amber from plastic?

The most reliable home test is the salt water float test: dissolve as much salt as possible in water, then drop in the stone. Real amber floats; most plastics sink. Under UV light, real amber typically shows blue-green fluorescence. A hot needle applied to an inconspicuous spot will smell like pine resin if it is real amber.

What is moldavite?

Moldavite is a natural glass (tektite) formed about 15 million years ago when a meteorite impact in what is now Germany melted rock and ejected it into the atmosphere. It cooled as it fell, forming green glassy pieces found mainly in the Czech Republic. It is amorphous, like obsidian, but of extraterrestrial origin.

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Conclusion

Amorphous gemstones remind us that beauty in the mineral world does not require perfect atomic order. Opal's play of color, obsidian's volcanic glass, amber's ancient inclusions, and jet's velvety black all arise from disordered atomic structures that defy the rules of crystallography. Understanding what makes these gems amorphous deepens our appreciation for the extraordinary diversity of nature's gem-forming processes.