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Crystal Cubes: Natural Pyrite, Fluorite and Halite Cubes Versus Cut Stone Cubes

By Digital Towns Crystals · Last reviewed October 8, 2026

Short answer: a "crystal cube" can be three quite different things. A grown cube formed with six faces as the mineral crystallized, the way pyrite, fluorite, halite and galena often do. A cleavage cube is a broken piece of a mineral, such as halite or galena, that splits along three planes at right angles. A cut cube is a lapidary shape ground from any stone, often jasper, agate or selenite. Natural cubes are valued for sharp faces and luster; cut cubes for pattern and precision.

Which minerals actually grow as cubes?

Only minerals in the isometric (cubic) crystal system can grow true cubes, and even they often choose another form. The four that buyers meet most:

Mineral Mohs hardness Specific gravity Cube habit Cleavage
Pyrite (FeS2) 6 to 6.5 4.9 to 5.2 cubes, pyritohedra, octahedra; faces often striated none
Fluorite (CaF2) 4 3.0 to 3.3 cubes and octahedra, sometimes combined perfect, octahedral
Halite (NaCl) 2 to 2.5 2.1 to 2.6 cubes, often hoppered perfect, cubic
Galena (PbS) 2.5+ 7.4 to 7.6 cubes and octahedra perfect, cubic

The cleavage column matters as much as the habit column. Halite and galena break into cubes; fluorite and pyrite do not. That single fact sorts most of the "natural cube" questions below.

Pyrite

Minerals.net lists pyrite as isometric with hardness 6 to 6.5, specific gravity 4.9 to 5.2 and no cleavage, and names the Ampliación a Victoria Mine at Navajún, La Rioja, Spain, as the place where large, perfect cubic crystals are mined in abundance, frequently embedded in a light brown matrix. Geology.com adds that pyrite crystals often have striated faces. Because pyrite has no cleavage, a pyrite cube with flat, square faces grew that way. Some Spanish cubes are so sharp that buyers suspect they were cut; the striations are the evidence that they were not, since a polishing wheel would erase them. More is in the pyrite guide.

Peruvian pyrite reaches the market in quantity too: Minerals.net notes that most amateur collector pyrite comes from Peruvian localities. Peruvian clusters often show many intergrown cubes rather than one isolated crystal. Letter grades such as "C grade" are each seller's own scale, so ask what the letter means for that piece.

Fluorite

Minerals.net describes fluorite crystals as most commonly octahedral and cubic, sometimes combining the two, and notes frequent penetration twins in which one cube grows through another. Its cleavage, though, is octahedral: parallel to the octahedron faces. So a fluorite cube was grown, while a smooth fluorite octahedron may well have been cleaved from a larger piece. Among U.S. sources, Minerals.net singles out Hardin County, Illinois, for producing more collectible specimens than anywhere else in the country, and lists the Elmwood Mine at Carthage, Smith County, Tennessee, among other excellent deposits. The fluorite guide covers its colors.

Halite and galena

Halite grows as cubes, often distorted by hopper growth, and splits into cubic fragments; Geology.com describes its cleavage as perfect in three directions at right angles. Minerals.net warns that artificial halite is easily grown by evaporating a saturated brine, that hopper cubes can result, and that a few halite specimens on the market are lab-grown in exactly this way. It also explains that the pink and peach color of much dry-lake halite comes from bacteria, not from impurities in the salt itself. See the halite guide.

Geology.com describes galena the same way: perfect cubic cleavage in three directions meeting at 90 degrees, with crystals that are usually cubes or octahedra. Galena is lead sulfide, and with a specific gravity of 7.4 to 7.6 a small cube feels startlingly heavy.

Is it a grown cube or a cleavage block?

With halite and galena, a dealer can produce neat cubes from a broken mass in seconds by tapping along the cleavage. These are real minerals but not crystals in the collector's sense. Telling them apart:

  1. Faces. Grown faces often carry growth features: striations, hopper steps, small etch pits, slightly raised edges. Cleavage faces are flat and mirror-bright, sometimes with fine steps where the split jumped between planes.
  2. Edges and corners. Grown cubes in a cluster meet their neighbors and the matrix; a cleavage block is isolated, with all twelve edges equally sharp.
  3. Attachment. A cube still sitting on matrix, like a Navajún pyrite in its host rock, almost certainly grew there. A loose cube needs a closer look.
  4. The mineral's cleavage. If the mineral has no cubic cleavage (pyrite, fluorite), a cube cannot be a cleavage fragment.

Why are apophyllite "cubes" not really cubes?

Apophyllite from the Deccan Traps of India, which Minerals.net names as the source of by far the largest crystals, is widely sold as "cubes," but Minerals.net places the common apophyllite species in the tetragonal system, not the isometric one. It describes the crystals as most often cubic in shape with distinctive triangular corners, and commonly pseudocubic or rectangular. In a tetragonal crystal one axis differs from the other two, so the top of a block-shaped apophyllite is not equivalent to its four sides. Minerals.net notes the luster is pearly on cleavage surfaces and the crystals are almost always striated. The trade word "cube" describes the look, not the symmetry. The apophyllite guide covers these specimens.

How is a cut stone cube made, and what is a cut corner cube?

A cut cube is ground from a sawn block until all six faces are square and every edge meets at a right angle. Wolfram MathWorld describes the cube as the Platonic solid made of six square faces meeting at right angles, with eight vertices and 12 edges. The lapidary challenge is to keep all three dimensions equal while keeping each face flat and every angle square. Errors show quickly: a cube that rocks on a table, or a face whose reflection bends.

A cut corner cube has one corner ground off, leaving a small extra face. The point is display: the extra face catches the light at a different angle and gives a patterned stone an additional view.

Cut cubes favor patterned, opaque stones. Jasper and agate (chalcedony, hardness about 7) keep crisp edges. Selenite cubes are a different matter: the International Gem Society gives gypsum a hardness of 2 and specific gravity of 2.32, so a selenite cube is light and marks easily at the edges.

Checking a cube's size by weight

A cube's volume is the side length cubed, and weight is volume times specific gravity. A selenite cube 51 mm on a side holds about 133 cm³; at 2.32 that is roughly 308 g. A 57 mm jasper cube holds about 185 cm³, or roughly 480 g at a specific gravity of 2.6, less whatever the cut corner removed. A listed weight far below the estimate means the cube is smaller than stated or has a large corner cut away.

What to look for when you buy a crystal cube

For a natural cube: sharp, undamaged edges; faces with luster and, on pyrite, intact striations; a matrix that shows the cube grew in place; and an honest label on grade and repairs. For halite, ask whether the piece is natural or regrown. For a cut cube: equal dimensions, flat faces, square corners, and a pattern worth looking at on every face.

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Frequently asked questions

Are pyrite cubes natural or cut?

Natural. Pyrite grows as cubes, and the best come from Navajún, Spain, often still set in their host rock. The fine parallel striations on the faces are growth features that cutting would remove. Pyrite has no cleavage, so it cannot be split into neat cubes either.

Why do some fluorite pieces look like octahedrons instead of cubes?

Fluorite grows as both cubes and octahedra, but its cleavage runs parallel to the octahedron faces. Many smooth octahedra on the market were split from larger crystals along those planes. A sharp fluorite cube, by contrast, grew that shape.

Is apophyllite cubic?

No. The common apophyllite species crystallize in the tetragonal system. The crystals often look like cubes with clipped triangular corners, which is why dealers call them cubes, but one crystal axis differs from the other two.

What is a cut corner cube?

A lapidary cube with one corner ground away to make a small extra face. It adds another viewing angle and catches light differently, which suits patterned stones such as jasper and agate. It is a decorative cut, not a natural crystal form.

Can halite cubes be fake?

They can be regrown. Halite crystallizes easily when a saturated brine evaporates, and some cubes and hoppers sold as specimens were grown that way. Natural pieces usually come with a locality such as Searles Lake, California; ask the seller if none is given.

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