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Pseudomorphs: How One Mineral Takes the Shape of Another

By Digital Towns Crystals · Last reviewed October 8, 2026

Short answer: a pseudomorph ("false form") is a specimen whose outside shape belongs to one mineral while its substance is another. It forms when a crystal is dissolved and replaced, coated and then removed, or chemically altered while its outline survives. Labels write it as "new mineral after old mineral": malachite after azurite, goethite after pyrite, quartz after fluorite. The giveaway is a crystal shape the mineral inside could never grow on its own.

What does "after" mean on a mineral label?

It is a fixed piece of collector grammar. "Goethite after pyrite" means goethite is what the specimen is made of now, and pyrite is the mineral whose crystal shape it kept. The Tulane mineralogy notes by Stephen Nelson define pseudomorphism as the existence of a mineral that has the appearance of another mineral, formed when a mineral is altered so that its internal structure and chemical composition change but its external form is preserved.

The Mindat glossary gives the textbook case in one line: cubes of goethite after pyrite, resulting from oxidation of the ferrous sulfide to ferric oxyhydroxide. Pyrite grows cubes; goethite does not. A brown cube that streaks yellow-brown is therefore a record of a mineral that is no longer there.

How does one mineral replace another?

Nelson separates three mechanisms. Real specimens often mix them, but the distinctions explain what you see.

Substitution: atom for atom

The old mineral dissolves while the new one precipitates in its place, at nearly the same moment, so the shape and sometimes the internal detail survive. Nelson's two examples are wood replaced by quartz to make petrified wood, and fluorite crystals replaced by quartz so that the quartz looks isometric and is described as pseudomorphed after fluorite. A quartz "cube" is impossible as a grown crystal, so its shape alone proves replacement.

Encrustation: a coat, then an empty mold

A thin crust of a new mineral forms on the surface of a crystal, and then the original crystal dissolves away. Nelson notes that the crust then carries casts of the original mineral's form. Encrustation pseudomorphs are often hollow, or filled later by a third mineral, and the walls can be paper thin.

Alteration: partial replacement

Only some of the original is removed and only some of the new mineral grows, so the space once filled by the first mineral ends up partly made of the second. Nelson lists serpentine after olivine or pyroxene, anhydrite after gypsum, limonite after pyrite and anglesite after galena. These are the specimens that show a core of fresh mineral inside a rind of the new one when broken.

Where do pseudomorphs form, and what drives them?

Mostly where water meets an unstable mineral. Three settings produce most collector material.

Setting Chemistry Classic pseudomorph Locality noted in sources
Weathering zone of sulfide ores Oxygen and water oxidize sulfides Goethite or limonite after pyrite Rio Marina, Elba; Porter, North Carolina
Copper oxidation zones Copper carbonates swap as CO2 and water change Malachite after azurite or cuprite Tsumeb, Namibia; Chessy, France
Buried wood and shell Silica-rich groundwater Chalcedony after wood or coral Tampa Bay and Tarpon Springs, Florida
Evaporite and hydrothermal pockets Soluble minerals dissolve, silica arrives Chalcedony or quartz after carbonates and sulfates Described as not uncommon by Akhavan

Iron: the rusting cube

Pyrite is iron sulfide, and near the surface it is unstable. Oxygenated water attacks it, the sulfur leaves as sulfate, and iron is redeposited as ferric oxyhydroxide in the same space. Minerals.net notes that limonite commonly forms as a pseudomorph over pyrite, marcasite and siderite, and that these pseudomorphs are recognizable by their partly rusted look and yellow staining. It names Rio Marina on Elba and Arenalejos, Carratraca, Spain, as sources of excellent limonite after pyrite, plus limonite after calcite from the Silver Hill mine in Pima County, Arizona. For goethite, Minerals.net cites branching pseudomorphs after marcasite from the Farfara Oasis in Egypt's White Desert and elongated goethite after gypsum from Santa Eulalia, Chihuahua.

Copper: blue to green

In copper deposits the replacement runs the other way in color. Minerals.net's malachite page states that large individual malachite crystals are very rare and are usually pseudomorphs after azurite or cuprite. It singles out Tsumeb, Namibia, for malachite after azurite and Chessy, in France's Rhône-Alpes, for malachite after cuprite in octahedral and even dodecahedral crystals. Because malachite itself grows as fibers and crusts, any sharp, blocky green crystal is worth a second look; it is very likely a malachite pseudomorph of something else.

The same change happens slowly on a much smaller scale. Geology.com explains that azurite pigment in medieval paintings slowly weathers to malachite with exposure to air and light, which is why some old blue skies now look green. More on that instability is in our azurite guide.

Native metals can play the replacing role too. Minerals.net's copper page mentions odd pseudomorphs of copper after aragonite from Corocoro, La Paz, Bolivia: metallic copper in the six-sided outlines of twinned aragonite.

Silica: fossils in stone

Silica replacement is how most "fossil" gems form. Minerals.net's chalcedony entry notes that chalcedony forms pseudomorphs after organic material, with petrified wood the best-known example, and lists chalcedony moldings and replacements of coral from western Florida, including Tampa Bay and Tarpon Springs. A. C. Akhavan's Quartz Page adds that chalcedony pseudomorphs after water-soluble minerals, such as carbonates and sulfates, are not uncommon. Our petrified wood guide covers the wood case in detail.

Is tiger's eye really a pseudomorph?

This is the best-known open argument in the subject. Penn State reported in 2003 that since 1873, following a suggestion by Wibel, geologists had treated tiger's eye as quartz pseudomorphed after crocidolite fibers, and that the idea had endured in standard textbooks for 125 years. Peter Heaney and Donald Fisher examined the stone with optical microscopes, X-ray diffraction and transmission electron microscopy and concluded that the textures do not offer convincing evidence for quartz pseudomorphism and fit a discontinuous crack-seal mechanism instead.

The older view has not disappeared. Minerals.net's tiger's eye gemstone page still describes it as a pseudomorph of compact quartz after crocidolite. Akhavan records field evidence that once supported replacement (following a vein down from the surface, collectors meet tiger's eye, then hawk's eye a few meters lower, then unaltered crocidolite) but writes that his own thin sections make simultaneous growth of quartz and fibers look likely. Our tiger's eye guide has the optical side; here the point is that "pseudomorph" is an interpretation, and interpretations get revised.

What are the common misconceptions?

  • "Pseudomorph" means fake. It does not. A pseudomorph is entirely natural; only its shape is borrowed.
  • Any color change is a pseudomorph. A green coating on azurite is just a coating unless the green mineral has taken over the crystal's form.
  • Paramorphs are the same thing. The Mindat glossary sets paramorph apart as a separate term. The usual meaning is a change of structure without a change of chemistry, such as aragonite converting to calcite. Nelson notes that the aragonite to calcite change is a reconstructive transformation, which is why aragonite can survive for long periods as a metastable form.
  • The original mineral is always gone. Not in alteration pseudomorphs, which by Nelson's definition are only partly replaced. Broken specimens of limonite after pyrite can still show a bright sulfide core, and those cores keep reacting with air.

What to look for when you buy a pseudomorph

  1. A shape that does not belong. Cubes of a brown iron oxide, quartz shaped like fluorite, green crystals in azurite's tabular form. If the habit could be the mineral's own, the specimen may simply be a normal crystal.
  2. Both minerals named. A label that says only "malachite" for a sharp green crystal undersells it, and one that says "malachite after azurite" without a visible azurite habit oversells it.
  3. Streak and luster. Goethite after pyrite should streak yellow-brown, not the greenish-black of fresh pyrite.
  4. Fragility. Encrustation pseudomorphs can be hollow shells. Check for crushed or repaired faces in the photos.
  5. Locality. Pseudomorph value depends heavily on a named mine. Ask for it, and be skeptical of famous localities attached to generic material.

The pieces below are the common end of the subject: the minerals involved in copper replacement, and silica-replaced wood.

  • Azurite on Matrix: look along crystal edges and the matrix for green malachite spots, the first stage of the azurite to malachite change.
  • Malachite: banded, fibrous malachite, the mineral's own habit; useful for comparing against the blocky crystals that signal a pseudomorph.
  • Petrified Wood: small pieces of silica-replaced wood; look for preserved growth rings and grain under a loupe.
  • Petrified Wood Sphere: Indonesian petrified wood, listed as roughly 20 million years old, where the replaced structure shows across a polished surface.
  • Tiger's Eye Crystal Polished Slice Natural Chatoyant: the disputed case, worth owning as an example of a textbook pseudomorph that may not be one.

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Frequently asked questions

Is petrified wood a pseudomorph?

Yes, in the mineralogical sense. The original wood was replaced, mostly by silica, while its cell structure and growth rings were preserved, which is the substitution mechanism. Most collectors simply call it a fossil, and both descriptions are correct.

How can you tell malachite after azurite from ordinary malachite?

By shape. Ordinary malachite grows as fibrous bands, crusts and botryoidal masses. Malachite after azurite keeps azurite's sharp, tabular or prismatic crystal outlines, sometimes with a blue core left inside. A crisp green crystal with flat faces is the clue.

Are pseudomorphs worth more than normal crystals?

Good ones often are, because they are scientifically interesting and the classic localities are worked out. Value depends on sharpness of the borrowed form, how complete the replacement is, size and a named locality. Poorly formed or unlabeled pieces sell like the mineral they are made of.

What is the difference between a pseudomorph and a cast?

A cast or mold is a void or infill left after a crystal dissolved, without the new mineral having taken over the crystal itself. An encrustation pseudomorph sits between the two: a shell of new mineral that copied the old crystal's faces before the original disappeared.

Can a pseudomorph keep changing?

Some can. Malachite replacing azurite and limonite replacing pyrite are both reactions with air and water, so specimens that still contain the original mineral should be kept dry and away from heat.

Sources