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. Streak: white.
- 3Read the lusterHold the specimen under a strong light. Luster: pearly.
- 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 3-3.5 vs. 2).
How to tell apart: Calcite is the harder of the two (Mohs 3 vs. 2); luster reads pearly on Fishtail Selenite and vitreous; pearly on cleavages and {0001} on Calcite.
How to tell apart: Celestine is the harder of the two (Mohs 3-3.5 vs. 2); luster reads pearly on Fishtail Selenite and vitreous, pearly on cleavages on Celestine.
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
Where rockhounds find fishtail selenite
Classic worldwide localities
- Naica Mine, Mexico
- Red River Floodway, Canada
- Oklahoma, USA
- Lubin, Poland
Field-hunting tip
Where it forms: evaporite deposits and clay beds. If you start seeing calcite, aragonite, sulfur in float, you are in the right ground. Typical habit: twinned prismatic crystals with a characteristic V-shape. Train your eye for that shape before scanning the outcrop.
Common questions
How do you identify fishtail selenite?+
Where is fishtail selenite found?+
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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