Short answer: both are gypsum, calcium sulfate with two waters, and both sit at Mohs 2. Selenite is the transparent, crystalline variety that peels into thin, glassy sheets. Satin spar is the fibrous variety: milky, silky, with a band of light that slides along it as you turn it. Nearly every white wand, tower, lamp, bowl, sphere and charging plate sold as "selenite" is satin spar. Look through it and tilt it: if you cannot read through it and the shine moves in a stripe, it is satin spar.
Why is almost every "selenite" wand really satin spar?
Because of how the mineral names are defined, and how the carvings are made. Mineral references use the two words for two different growth habits of the same species. Minerals.net defines selenite as the transparent and colorless (or very lightly colored) variety of gypsum that forms in distinct crystals (Minerals.net, selenite). Satin spar, on the same site, is simply the fibrous variety of gypsum (Minerals.net, satin spar).
Now look at a typical shop "selenite" log: opaque white, cut square or round, with a sheen that runs along its length. That object is made of parallel fibers, not of a clear crystal, so by those definitions it is satin spar. The International Gem Society describes satin spar as a massive, fibrous variety of gypsum that can be carved or cabbed, and notes that fibrous material could yield large carvings up to several pounds (International Gem Society). Clear selenite, by contrast, splits along its cleavage the moment a saw or lathe touches it, which is why it is sold as natural blades and clusters rather than turned into spheres.
None of this makes satin spar fake. It is real gypsum, and Geology.com notes that the varieties known as satin spar and alabaster are used for ornamental purposes, with their low hardness limiting durability (Geology.com, gypsum). The issue is only the label. A seller who writes "satin spar (gypsum)" is giving the buyer more information, not less.
How the two compare, property by property
| Property | Selenite | Satin spar |
|---|---|---|
| Species | Gypsum, CaSO4·2H2O | Gypsum, CaSO4·2H2O |
| Habit | Distinct tabular or bladed crystals, often twinned | Parallel fibers filling veins |
| Transparency | Transparent, "water-clear" when fine | Translucent to nearly opaque, milky |
| Luster | Glassy faces, pearly on cleavage | Silky, satiny |
| Optical effect | None (flashes from flat cleavage planes) | Chatoyancy, a moving band or cat's eye |
| How it breaks | Peels into flexible sheets along one perfect cleavage | Splinters into fibers along its length |
| Mohs hardness | 2 (1.5 in some directions) | 2 |
| Specific gravity | About 2.3 | About 2.3 |
| Typical market form | Natural blades, clusters, fishtail twins | Wands, towers, logs, spheres, bowls, plates, lamps |
| Typical price driver | Clarity, size, crystal form, locality | Size, cut quality, color |
Data from Minerals.net, Webmineral, Gemdat and the International Gem Society.
The bottom two thirds of that table matter most. Every line that would normally separate two minerals (hardness, density, chemistry) is identical here, because this is one mineral grown two ways.
Five tests that separate them
1. Read print through it
Lay the piece on a page of text. A good selenite blade lets you read through it, sometimes with a slight doubling at the edges. Satin spar blurs the text into a glow at best, and a thick log blocks it entirely. If light passes through only along the length of the piece, like a fiber-optic bundle, you have satin spar.
2. Rotate it under one bulb
Hold the piece under a single light and roll it slowly. On satin spar a bright stripe runs across the fibers and slides as you turn it; Minerals.net notes the variety is sometimes cut into cabochons for its strong cat's eye effect. Selenite does something different: whole flat faces light up and go dark at once as the cleavage planes catch the bulb, and those planes look pearly. The International Gem Society traces the name selenite to the Greek selene, "moon," because of that pearly luster on cleavage surfaces.
3. Look at the broken end
Turn a wand or a raw chunk end-on and use a 10x loupe. Satin spar shows a brush of fine threads, and a damaged end looks frayed. Webmineral lists a fibrous fracture for gypsum, meaning thin, elongated breaks along the crystal direction. Selenite shows stacked layers, like the edge of a closed book, with small stair-steps where sheets have lifted.
4. The flake test (on raw material only)
On an unpolished blade or a hidden corner, press a fingernail under the edge of a layer. Selenite lifts a thin, clear sheet that bends without snapping, because its main cleavage is micaceous and slim crystals are slightly flexible, according to Minerals.net. Satin spar will not give you a sheet. It sheds a few silky fibers instead. Never try this on a finished sphere or tower.
5. Check the tests that do not work
Hardness and weight will not help. Both scrape with a fingernail (Geology.com lists a fingernail at 2 to 2.5 on its Mohs hardness page), and both sit around 2.3 on a specific gravity scale. They are useful only for ruling out impostors. Minerals.net notes that the term satin spar may occasionally also describe a fibrous form of calcite or aragonite; a fingernail will not mark those, and they fizz in acid where gypsum does not.
How does each one grow?
Gypsum is an evaporite mineral, most commonly found in layered sedimentary deposits with halite, anhydrite, sulfur, calcite and dolomite, according to Geology.com. The two varieties come from different settings inside that story.
Satin spar is a vein filling. Gemdat describes it as filling veins in rock formations with parallel fibrous crystals stretching across the vein from side to side. Picture a crack in mudstone or clay that slowly opens while gypsum-rich water seeps in: the fibers grow across the gap, so a vein a hand thick becomes a slab of fibers a hand long. Cut that slab lengthwise and you have a wand blank.
Selenite grows as separate crystals with room to develop faces. Minerals.net notes that many gypsum crystals form in clay beds as floaters, growing without attachment to a matrix, and that crystals frequently twin into fishtail or swallowtail shapes. The extreme case is the Cave of the Crystals at Naica, Chihuahua, Mexico, where the International Gem Society reports the largest crystal found so far measures 39 feet by 13 feet and weighs over 13 tons. The cave was re-flooded in 2015.
Curved "ram's horn" gypsum sits between the two. Minerals.net notes that crystals and fibrous masses may be curved, sometimes severely, into the forms collectors call ram's horns.
Where does each come from?
| Variety | Well-known localities | Notes |
|---|---|---|
| Selenite | Naica and Santa Eulalia, Chihuahua, Mexico | Naica produced the giant crystals; Santa Eulalia has produced outstanding selenite |
| Selenite | Ellsworth, Mahoning Co., Ohio; Hanksville, Utah | Ohio gives stand-alone crystals; Utah has produced large crystals |
| Selenite | Red River Floodway, Winnipeg, Manitoba | Golden crystals, some fluorescent |
| Satin spar | Derbyshire, England | Classic fibrous specimens pictured by Geology.com |
| Satin spar | Niagara Falls region; Egypt | Historic tourist cabochons called "Niagara spar" and "Pharaoh's Eggs" |
The selenite rows come from Minerals.net and the International Gem Society. The satin spar history comes from the IGS, which cites the mineralogist George F. Kunz on egg-shaped satin spar cabochons from Egypt and Niagara. Most carved satin spar on today's market carries a Moroccan origin label, as our own spheres do; ask the seller if the locality matters to you.
Does the difference change price or care?
Price, yes. Satin spar is a bulk carving material, so you pay mostly for size and finish: a small sphere costs a few dollars, a long polished log or lamp more. Selenite is priced as a mineral specimen. Clarity, sharp terminations, twinning, an attached matrix and a famous locality all add value, and Minerals.net ranks selenite and desert rose among the gypsum forms most popular with collectors.
Care is similar but the failure modes differ. Both scratch easily and both dislike heat; the IGS warns never to use mechanical cleaners that use heat or vibration on any gypsum. Satin spar tends to fray and snag at the ends and lose its silky polish, while selenite tends to split along cleavage if dropped. The selenite storage guide covers handling and humidity in detail.
Which should you buy for what?
- For a glowing shelf piece, lamp or a large carving: satin spar. Only fibrous material comes in carvable logs, and its chatoyant band is the whole point of the object.
- For a mineral collection: selenite. A clear blade, a fishtail twin or a cluster on matrix shows what gypsum crystals actually look like.
- For a teaching pair: one of each, side by side, so the reading-through test and the moving-band test can be seen in seconds.
In crystal traditions, the "selenite" associated with clarity and with charging plates is almost always satin spar in practice; the main selenite guide covers those associations and the other gypsum forms, including desert rose.
Shopping selenite and satin spar we stock
Read listings with the table above in mind. A listing that shows a white, opaque piece with a silky stripe and calls it selenite is selling satin spar; that is fine, but it should be priced as a carving, not as a crystal specimen. For true selenite, look for photos taken against a dark background where you can see through the piece, and for a stated locality and weight.
- Clear Selenite Gypsum Cluster, 11.27 lb, Morocco: a large cluster of genuinely crystalline gypsum, the variety the word selenite properly names.
- Ram's Horn Selenite mineral specimen: the curved gypsum habit Minerals.net describes, an inexpensive way to own a natural, uncarved form.
- Selenite sphere, 6 cm, Morocco: a 250 to 300 gram sphere; roll it under a lamp and the satin spar band shows at once.
- Selenite squared log: a 1,036 gram cut log with the silky, striated surface that marks fibrous satin spar.
- 4" peach selenite moon bowl: iron-tinted fibrous gypsum carved into a crescent bowl, a good example of satin spar as a carving stone.
- Golden Selenite (UT): a small, low-cost piece labeled Utah; check it with the read-through test to see which habit you have.
Digital Towns Market
Selenite and satin spar in stock
Frequently asked questions
Is satin spar real selenite?
Satin spar is real gypsum, the same mineral as selenite, but it is a different variety. Selenite means clear, crystalline gypsum; satin spar means fibrous gypsum. Most wands, towers and lamps sold as selenite are satin spar, which is genuine and attractive but not crystalline selenite in the mineralogical sense.
Why does my selenite wand have a moving line of light?
That band is chatoyancy, the same cat's eye effect seen in tiger's eye. It comes from light reflecting off thousands of parallel gypsum fibers, and it is the signature of satin spar. Clear selenite does not show a moving band; it flashes from flat, pearly cleavage faces instead.
Can you tell selenite from satin spar by weight or hardness?
No. Both are gypsum, so both have a Mohs hardness of about 2 and a specific gravity around 2.3. A fingernail scratches either one. Use transparency, the moving-band test and the way the piece breaks (sheets versus fibers) instead. Hardness and acid only help rule out fibrous calcite or aragonite.
Is satin spar cheaper than selenite?
Usually, yes, piece for piece. Satin spar is a carving material sold by size and finish, so even large logs and spheres stay affordable. Fine selenite is sold as a specimen, and clarity, twinning, matrix and locality can push a single crystal far above the price of a large satin spar carving.
Why are there no selenite spheres made of clear crystal?
Clear selenite has a perfect, easy cleavage and splits into sheets under the stress of cutting, so it does not survive being turned into a sphere or tower. Fibrous satin spar holds together well enough to carve. That is why virtually every gypsum sphere, tower and bowl is satin spar.
Sources
- Minerals.net, Selenite (gypsum variety): https://www.minerals.net/mineral/gypsum/selenite
- Minerals.net, Satin Spar (gypsum variety): https://www.minerals.net/mineral/gypsum/satin-spar
- Minerals.net, Gypsum: https://www.minerals.net/mineral/gypsum.aspx
- International Gem Society, Gypsum Value, Price, and Jewelry Information: https://www.gemsociety.org/article/gypsum-jewelry-and-gemstone-information/
- Geology.com, Gypsum: https://geology.com/minerals/gypsum.shtml
- Geology.com, Mohs Hardness Scale: https://geology.com/minerals/mohs-hardness-scale.shtml
- Webmineral, Gypsum Mineral Data: http://webmineral.com/data/Gypsum.shtml
- Gemdat, Satin Spar: https://www.gemdat.org/gem-8573.html