Epididymite is a rare beryllium silicate mineral that typically forms white, thin tabular or platy crystals. It is most famously found in the alkaline pegmatites of Mont Saint-Hilaire, where it is often associated with other rare beryllium species and zeolites. It is dimorphous with eudidymite, and distinguishing between the two often requires laboratory testing or precise crystallographic analysis.
Is this epididymite?
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 epididymite with a known reference. Epididymite sits at Mohs 5.5, 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 on cleavages, vitreous on fractures.
- 4Match the color rangeCompare against the expected color range: white, colorless, gray, yellow, blue.
- 5Look at form & habitCrystal system: orthorhombic. Typical habit: tabular crystals, platy, radiating aggregates.
Often confused with
Epididymite vs. its common look-alikes, and how to tell them apart in the field.
How to tell apart: Luster reads pearly on cleavages, vitreous on fractures on Epididymite and vitreous, pearly on cleavage on Eudidymite.
How to tell apart: Luster reads pearly on cleavages, vitreous on fractures on Epididymite and vitreous, typically pearly on cleavages on Albite.
Often found alongside epididymite
Minerals reported to co-occur with epididymite. Spotting these in float or country rock is a strong cue you are in the right ground.
All properties
- Chemical formula
- NaBeSi₃O₇(OH)
- Mohs hardness
- 5.5
- Density
- 2.55-2.61 g/cm³
- Streak
- White
- Luster
- Pearly on cleavages, vitreous on fractures
- Transparency
- Transparent to translucent
- Crystal system
- Orthorhombic
- Crystal habit
- Tabular crystals, platy, radiating aggregates
- Cleavage
- Perfect on {001}, distinct on {100}
- Rarity
- Rare
- Uses
- Collector
- Host rock
- A late-stage mineral in nepheline syenite pegmatites
Where rockhounds find epididymite
Classic worldwide localities
- Quincy, Norfolk Co., Massachusetts, USA
- Arø Island, in the Langesundsfjord, and in the Vevja quarry, Tvedalen, Norway
- Kola Peninsula, in the Lovozero massif, on Mts. Karnasurt and Alluaiv, and at Kuivchorr, and in the Khibiny massif, Russia
- Murun massif, southwest of Olekminsk, Yakutia, Russia
- Věžná, Czech Republic
- Mont Saint-Hilaire and near Saint-Amable, Quebec, Canada
Field-hunting tip
Where it forms: A late-stage mineral in nepheline syenite pegmatites. If you start seeing albite, aegirine, eudialyte in float, you are in the right ground. Typical habit: tabular crystals, platy, radiating aggregates. Train your eye for that shape before scanning the outcrop.
Common questions
How do you identify epididymite?+
Where is epididymite found?+
Is epididymite safe to handle?+
What rocks look like epididymite?+
What minerals are found with epididymite?+
What kind of rock does epididymite form in?+
What is epididymite used for?+
Find epididymite on the map
RockHoundR shows mapped rockhounding spots, access rules, and lets you log every find.
