Fishtail Selenite refers to the cyclic contact twinning of gypsum crystals that form distinct V-shaped or 'fishtail' morphologies. These crystals are highly fragile and prized by collectors for their geometric symmetry and excellent transparency, often found in clay-rich evaporite deposits.
Is this fishtail selenite?
5-step field checkRun through these checks against the specimen in your hand. The more boxes tick, the more confident the ID.
- 1Test the hardnessTry to scratch fishtail selenite with a known reference. Fishtail Selenite sits at Mohs 2, softer than the next harder reference, harder than the previous one.
- 2Check the streakDrag the specimen across an unglazed porcelain plate. Fishtail Selenite leaves a white streak.
- 3Read the lusterHold the specimen under a strong light. Fishtail Selenite typically shows a pearly luster.
- 4Match the color rangeCompare against the expected color range: colorless, white, yellowish, gray.
- 5Look at form & habitCrystal system: monoclinic. Typical habit: twinned prismatic crystals with a characteristic V-shape.
Often confused with
Fishtail Selenite vs. its common look-alikes, and how to tell them apart in the field.
How to tell apart: Anhydrite is the harder of the two (Mohs 3.5 vs. 2); luster reads pearly on Fishtail Selenite and vitreous on Anhydrite.
How to tell apart: Calcite is the harder of the two (Mohs 3 vs. 2); luster reads pearly on Fishtail Selenite and vitreous on Calcite.
How to tell apart: Celestite is the harder of the two (Mohs 3-3.5 vs. 2); luster reads pearly on Fishtail Selenite and vitreous on Celestite.
Often found alongside fishtail selenite
Minerals reported to co-occur with fishtail selenite. Spotting these in float or country rock is a strong cue you are in the right ground.
All properties
- Chemical formula
- CaSO₄·2H₂O
- Mohs hardness
- 2
- Density
- 2.31-2.33 g/cm³
- Streak
- White
- Luster
- Pearly
- Transparency
- Transparent
- Crystal system
- Monoclinic
- Crystal habit
- Twinned Prismatic Crystals with A Characteristic V-shape
- Cleavage
- Perfect in One Direction
- Fluorescence
- Often Fluorescent White or Yellow Under SW and LW UV
- Rarity
- Common
- Uses
- Collector, Decorative
- Host rock
- Evaporite Deposits and Clay Beds
- Typical price
- $5-50 for small specimens, $100+ for large museum-grade clusters
Where rockhounds find fishtail selenite
Classic worldwide localities
- Naica Mine, Mexico
- Red River Floodway, Canada
- Oklahoma, USA
- Lubin, Poland
Field-hunting tip
Look in evaporite deposits and clay beds country, the host setting where fishtail selenite typically forms. If you start seeing calcite, aragonite, sulfur in float, you are in the right ground. Field specimens usually show a twinned prismatic crystals with a characteristic v-shape habit, so train your eye for that shape before scanning the outcrop.
Common questions
How do you identify fishtail selenite?+
Where is fishtail selenite found?+
How much is fishtail selenite worth?+
What rocks look like fishtail selenite?+
What minerals are found with fishtail selenite?+
What kind of rock does fishtail selenite form in?+
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Find fishtail selenite on the map
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