Digital TownsCrystalsShop crystals
  1. Crystals & Minerals
  2. How they form
  3. Mineraloids

Mineraloids: Why Opal, Obsidian and Amber Never Became Crystals

By Digital Towns Crystals · Last reviewed October 8, 2026

Short answer: a mineraloid is a natural solid that fails at least one part of the definition of a mineral, usually because its atoms never lock into a repeating crystal lattice or its composition is not fixed. Most form in one of four ways: melt cooled too fast to crystallize (obsidian, tektites), silica or iron settling out of water as a gel (opal, limonite), living things making the material (amber, jet), or a mixture too fine to call one species.

What makes something a mineral, and where do mineraloids fall short?

Geology.com lists five requirements most geologists use. A mineral must be naturally occurring, inorganic, solid, of definite chemical composition, and have an ordered internal structure. Tulane University's mineral notes define a mineraloid simply as a substance that satisfies some, but not all, of those parts.

The test cases are instructive:

Material Natural Inorganic Solid Definite composition Ordered structure Verdict
Ice Yes Yes Yes Yes (H2O) Yes Mineral
Liquid water Yes Yes No Yes No Not a mineral
Obsidian Yes Yes Yes No No Mineraloid
Opal Yes Yes Yes No (variable water) No Mineraloid
Amber Yes No Yes No No Organic mineraloid

Geology.com is explicit that pearls and wood are not minerals because organisms make them, and that rocks such as obsidian are mineraloids because they lack a definite composition and an ordered internal structure. Tulane makes the same point about glass: it can form naturally, it is solid, but its composition is not always the same and it has no crystalline structure.

Why should a buyer care? Because the missing lattice controls almost everything you notice in the hand: no cleavage, conchoidal fracture, lower hardness than crystalline cousins, and often instability over time.

Route one: what happens when melt cools too fast?

Crystals need time for atoms to migrate into position. Take the time away and a liquid freezes as glass.

Obsidian comes from silica-rich rhyolitic magma. Tulane's notes on magma give rhyolitic magma 65 to 75% SiO2, eruption temperatures of 650 to 800°C, and high viscosity, which already slows atoms down. The International Gem Society puts typical obsidian at about 66 to 72% SiO2, while basaltic glass carries only about 50%. Geology.com's obsidian article lists the cooling settings: the edges of lava flows and domes, the margins of sills and dikes, lava meeting water, and lava cooling while airborne.

Impact glass is the extreme case: rock flash-melted by an asteroid strike and quenched in flight or near the crater. The impact glass guide covers tektites, moldavite and desert glass.

Glass is not stable forever. Given time and heat, it slowly starts to crystallize. Geology.com explains that this begins at scattered points in the rock, growing radial clusters of white or gray cristobalite; cut and polished, that is snowflake obsidian. The same source adds that obsidian older than a few million years is rare, because the glassy rock is destroyed or altered by weathering, heat or other processes. A snowflake obsidian is a glass caught partway toward becoming a crystalline rock.

Route two: how can silica or iron set like a gel?

Opal grows from water, not melt, and at surface conditions. GIA's opal description describes rain soaking into ancient rock in places such as Australia's semi-desert outback, carrying dissolved silica downward, then evaporating in dry periods to leave silica in cracks and between sedimentary layers.

What forms is not quartz. Minerals.net gives opal's formula as SiO2·nH2O, with water usually between 6% and 10% of the weight, and reports that the electron microscope revealed in the 1960s that opal is built of tiny silica spheres. In precious opal those spheres are the same size and stacked in an orderly grid that diffracts light; in common opal they are jumbled. That means opal has order at the scale of spheres but no crystal lattice at the scale of atoms. Geology.com gives it a hardness of 5 to 6 and specific gravity of 2.0 to 2.2, both below quartz, because of the water. The opal guide covers drying, crazing and Welo hydrophane behavior.

Limonite is the iron version. Geology.com's limonite page explains that research showed material called limonite does not meet the definition of a mineral: it is a mineraloid made mainly of hydrous iron oxides, often intimately mixed with goethite and hematite. The word survives as a field term for rust-brown iron oxide that cannot be identified without lab work.

Route three: what about materials made by living things?

Amber is fossilized tree resin. GIA gives its chemistry as C10H16O, specific gravity 1.08 and Mohs hardness 2 to 2.5, and dates the oldest amber to the Upper Carboniferous, about 320 million years ago. The resin is not replaced by minerals as in petrified wood; IGS describes it changing into a polymer, essentially a natural plastic, through time and pressure. IGS also notes that preserved resin younger than 30 million years is classed as copal, and that amber's low density allows a simple check: in water saturated with salt to a density of about 1.13, amber floats while most plastics sink.

Jet is a black organic rock that forms when woody material is buried in sediment and coalified. According to Geology.com, the famous jet of Whitby, England was deposited in a saltwater swamp about 180 million years ago, then compressed and heated through the rank of lignite almost to sub-bituminous coal. It has a hardness of 2.5 to 4, specific gravity of only 1.3 to 1.4, and a brown streak.

How do the common mineraloids compare?

Mineraloid Route Composition Mohs Specific gravity
Obsidian Quenched melt SiO2 about 66 to 72% plus oxides 5 Typically 2.33 to 2.42
Opal Silica gel from water SiO2·nH2O 5 to 6 2.0 to 2.2
Amber Fossil resin Approximately C10H16O 2 to 2.5 1.08
Jet Coalified wood Variable, carbon rich 2.5 to 4 1.3 to 1.4
Limonite Iron gel and mixture Hydrous iron oxides Variable Variable

The pattern is useful for buyers: mineraloids are mostly light and mostly soft. A "jet" bead that feels as heavy as glass, or "amber" that sinks in strong brine, deserves a second look.

What do people get wrong about mineraloids?

  • "Mineraloid means fake or lesser." It is a classification, not a grade. Precious opal with full play-of-color is a mineraloid, and so is every piece of genuine amber.
  • "Obsidian is a crystal." Shops label it that way, but it has no lattice. Its sharp, curved conchoidal fracture is a direct result.
  • "Snowflakes are a different stone glued in." They are cristobalite that crystallized inside the glass itself.
  • "Ice is not a mineral because it melts." Geology.com notes that ice is a naturally occurring, inorganic solid with a definite composition and ordered structure. Liquid water is not; frozen water is.
  • "Amber is fossilized sap that turned to stone." Nothing in it was replaced by stone. It is still organic resin, chemically altered.

Choosing opal and obsidian pieces

Because mineraloids are soft and lack a lattice, their selling points are surface and inclusions rather than crystal faces. For obsidian, look for an even polish without pits, a clear sheen or snowflake pattern across the whole piece, and undamaged edges; glass chips easily. For opal, ask whether the piece is precious or common opal, keep it out of heat and sudden dryness, and for hydrophane material such as Welo opal expect the look to change when wet.

  • Snowflake Obsidian: an inexpensive tumbled example of glass partway through crystallizing, with cristobalite clusters in black glass.
  • Silver Obsidian Palmstones: polished palm stones that show the silvery sheen IGS attributes to needlelike inclusions in some obsidian; check for an even sheen across the face.
  • Mahogany Obsidian Crystal Tower Volcanic Glass Obelisk: a tower that displays the red-brown iron banding of the mahogany variety; inspect the point and base edges for chips.
  • Welo (Ethiopia) Opal: Ethiopian opal, the hydrophane material that soaks up water; buy it to see what the silica sphere structure does to light.
  • Pink Opal and Green Opal: colored opal at low prices; check the photos for play-of-color, and if there is none, you are buying common opal, the everyday form of the mineraloid.

Digital Towns Market

Opal and obsidian in stock

See all crystals and minerals

Frequently asked questions

What is a mineraloid?

A naturally occurring solid that meets some but not all of the requirements for a mineral. Most mineraloids lack an ordered crystal structure, a fixed chemical composition, or both, or they are made by living things. Opal, obsidian, amber, jet and limonite are standard examples.

Is obsidian a mineral?

No. Obsidian is volcanic glass that cooled too quickly for crystals to grow, and its composition varies. That makes it a mineraloid. Over geological time it slowly crystallizes, which is why obsidian older than a few million years is rare.

Is opal a mineral or a mineraloid?

Opal is a mineraloid. It is hydrous silica, SiO2·nH2O, with a variable water content and no crystal lattice. Precious opal does have an orderly stacking of tiny silica spheres, which is what produces its play-of-color.

Is amber a mineral?

No. Amber is fossilized tree resin, an organic material with an approximate formula of C10H16O and a specific gravity of about 1.08. It is grouped with organic gems such as jet and pearl rather than with minerals.

Are mineraloids less durable than crystals?

Often, yes. Amber is 2 to 2.5 on the Mohs scale, jet 2.5 to 4 and obsidian about 5. Opal can also crack if it dries out. They need more careful storage than quartz or other hard crystalline stones.

Sources