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.

Hardness

5-6

Mohs

Luster

Vitreous

Streak

White

Transparency

Translucent

mineralrare

Cuspidine

Hardness: 5-6
Luster: Vitreous
Streak: White
Transparency: Translucent

Is this cuspidine?

5-step field check

Run through these checks against the specimen in your hand. The more boxes tick, the more confident the ID.

1
Test the hardness
Try to scratch cuspidine with a known reference. Cuspidine sits at Mohs 5-6 — softer than the next harder reference, harder than the previous one.
2
Check the streak
Drag the specimen across an unglazed porcelain plate. Cuspidine leaves a white streak.
3
Read the luster
Hold the specimen under a strong light. Cuspidine typically shows a vitreous luster.
4
Match the color range
Compare against the expected color range: red, yellow, brown, colorless.
5
Look at form & habit
Crystal system: monoclinic. Typical habit: prismatic crystals, granular aggregates.

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: Ca₄Si₂O₇(F,OH)₂
Mohs hardness: 5-6
Density: 2.98 g/cm³
Colors: Red Yellow Brown Colorless
Streak: White
Luster: Vitreous
Transparency: Translucent
Crystal system: Monoclinic
Crystal habit: Prismatic Crystals, Granular Aggregates
Cleavage: Distinct On {100}
Fluorescence: Bright Yellow Under SW UV
Rarity: Rare
Uses: Collector, Research
Host rock: Metamorphosed Limestone, Contact Skarns
Typical price: $20-150 for thumbnail to small cabinet specimens

Where rockhounds find cuspidine

Classic worldwide localities

Monte Somma, Italy
Franklin, New Jersey, USA
Khibiny Massif, Russia
Skellefte District, Sweden

Field-hunting tip

Look in metamorphosed limestone, contact skarns country — that is the host setting where cuspidine typically forms. If you start seeing calcite, diopside, vesuvianite in float, you are in the right ground. Field specimens usually show a prismatic crystals, granular aggregates habit, so train your eye for that shape before scanning the outcrop.

Common questions

How do you identify cuspidine?+

Mohs hardness is 5-6. It typically shows a vitreous luster. The streak is white. Common colors include red, yellow, brown, colorless.

Where is cuspidine found?+

Notable localities include Monte Somma, Italy; Franklin, New Jersey, USA; Khibiny Massif, Russia; Skellefte District, Sweden.

How much is cuspidine worth?+

Typical asking prices fall in the range of $20-150 for thumbnail to small cabinet specimens. Quality, size, and provenance can move individual specimens well outside that range.

What rocks look like cuspidine?+

Cuspidine is most often confused with Monticellite, Wollastonite. A quick hardness test and a streak check separate the look-alikes faster than color alone.

What minerals are found with cuspidine?+

Cuspidine commonly co-occurs with Calcite, Diopside, Vesuvianite, Grossular. Spotting any of these in float or country rock is a useful trip signal.

What kind of rock does cuspidine form in?+

Cuspidine typically forms in metamorphosed limestone, contact skarns. Working float back to the host body is the standard way to chase a fresh occurrence.

What is cuspidine used for?+

Cuspidine is used in collector, research.

Find cuspidine on the map

RockHoundR shows mapped rockhounding spots, access rules, and lets you log every find.