Cuspidine is a rare nesosilicate typically found in contact metamorphic skarns associated with limestone. It is most easily identified by its characteristic bright yellow fluorescence under short-wave UV light and its elongated, prismatic crystal habit. It is highly sought after by mineral collectors for its unique fluorescent properties and occurrence in complex skarn assemblages.
Is this cuspidine?
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 cuspidine with a known reference. Cuspidine 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: white.
- 3Read the lusterHold the specimen under a strong light. Luster: vitreous.
- 4Match the color rangeCompare against the expected color range: colorless, green, white, light pink.
- 5Look at form & habitCrystal system: monoclinic. Typical habit: acicular, massive.
Often confused with
Cuspidine vs. its common look-alikes, and how to tell them apart in the field.
Often found alongside cuspidine
Minerals reported to co-occur with cuspidine. Spotting these in float or country rock is a strong cue you are in the right ground.
All properties
- Chemical formula
- Ca₄(Si₂O₇)F₂
- Mohs hardness
- 5-6
- Density
- 2.85-2.96 g/cm³
- Streak
- White
- Luster
- Vitreous
- Transparency
- Transparent to translucent
- Crystal system
- Monoclinic
- Crystal habit
- Acicular, massive
- Cleavage
- Very good on {001}; distinct on {110}
- Rarity
- Rare
- Uses
- Collector, Research
- Host rock
- In tu® ejecta (Monte Somma, Italy); in contact metamorphosed limestone (Franklin, New Jersey, USA); in melilite skarn (Dupezeh Mountain, Iraq)
Where rockhounds find cuspidine
Classic worldwide localities
- Franklin, Sussex Co., New Jersey, USA
- Alder district, Custer Co., Idaho, USA
- Crestmore, Riverside Co., California, USA
- Cascade Mountain, Adirondack Mountains, Essex Co., New York, USA
- Monte Somma and Vesuvius, Campania, and in the Alban Hills, as at Ariccia, Lazio, Italy
- Carlingford, Co. Louth, Ireland
Field-hunting tip
Where it forms: In tu® ejecta (Monte Somma, Italy); in contact metamorphosed limestone (Franklin, New Jersey, USA); in melilite skarn (Dupezeh Mountain, Iraq). If you start seeing augite, hornblende, diopside in float, you are in the right ground. Typical habit: acicular, massive. Train your eye for that shape before scanning the outcrop.
Common questions
How do you identify cuspidine?+
Where is cuspidine found?+
What rocks look like cuspidine?+
What minerals are found with cuspidine?+
What kind of rock does cuspidine form in?+
What is cuspidine used for?+
Find cuspidine on the map
RockHoundR shows mapped rockhounding spots, access rules, and lets you log every find.
