Neptunite is prized by collectors for its striking, square-prismatic black crystals that often occur in sharp, contrast-rich association with white natrolite and blue benitoite. It is most famous for specimens from the California Gem Mine where it forms distinct, elongated crystals. While chemically complex, its combination of high-luster black crystals and specific geological associations makes it a highly sought-after mineral specimen.
Is this neptunite?
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 neptunite with a known reference. Neptunite sits at Mohs 5-6, softer than the next harder reference, harder than the previous one.
- 2Check the streakDrag the specimen across an unglazed porcelain plate. Streak: cinnamon-brown.
- 3Read the lusterHold the specimen under a strong light. Luster: vitreous.
- 4Match the color rangeCompare against the expected color range: black, red.
- 5Look at form & habitCrystal system: monoclinic. Typical habit: crystalline, prismatic, tabular.
Often confused with
Neptunite vs. its common look-alikes, and how to tell them apart in the field.
Often found alongside neptunite
Minerals reported to co-occur with neptunite. Spotting these in float or country rock is a strong cue you are in the right ground.
All properties
- Chemical formula
- KNa₂LiFe₂Ti₂Si₈O₂₄
- Mohs hardness
- 5-6
- Density
- 3.19-3.23 g/cm³
- Streak
- Cinnamon-brown
- Luster
- Vitreous
- Transparency
- Translucent
- Crystal system
- Monoclinic
- Crystal habit
- Crystalline, prismatic, tabular
- Cleavage
- Perfect on {110}
- Rarity
- Rare
- Uses
- Collector, Specimen
- Host rock
- In natrolite veins cutting a glaucophane schist inclusion in a serpentinite body (San Benito Co., California, USA)
Where rockhounds find neptunite
Classic worldwide localities
- Gem mine and Mina Numero Uno, San Benito Co., California, and at Point of Rocks, Colfax Co., New Mexico, USA
- Inagli massif, 30 km west of Aldan, Yakutia, and in the Khibiny and Lovozero massifs, Kola Peninsula, Russia
- Khan-Bogdinskii granitic massif, Gobi, Mongolia
- Dara-i-Pioz massif, Alai Range, Tien Shan, Tajikistan
- Barnavave, near Carlingford, Co. Louth, Ireland
- Mont Saint-Hilaire, Quebec, and Seal Lake, Labrador, Newfoundland, Canada
Field-hunting tip
Where it forms: In natrolite veins cutting a glaucophane schist inclusion in a serpentinite body (San Benito Co., California, USA). If you start seeing eudialyte, arfvedsonite, aegirine in float, you are in the right ground. Typical habit: crystalline, prismatic, tabular. Train your eye for that shape before scanning the outcrop.
Common questions
How do you identify neptunite?+
Where is neptunite found?+
What rocks look like neptunite?+
What minerals are found with neptunite?+
What kind of rock does neptunite form in?+
What is neptunite used for?+
Find neptunite on the map
RockHoundR shows mapped rockhounding spots, access rules, and lets you log every find.
