Ambrinoite is a rare potassium arsenic antimony oxide, typically cinnabar-red. It occurs in an evaporite deposit probably formed by highly alkaline, low-temperature hydrothermal fluids. Associated minerals include sulfur, orpiment and gypsum. It was first described from the Cumbë Sûrdë quarry, Signols, Oulx, Upper Susa Valley, Torino, Piedmont, Italy.

Hardness

Mohs

Luster

Vitreous to resinous

Streak

Reddish

Transparency

Transparent

mineralrareAeschynite Group family

Ambrinoite

Hardness: <2
Luster: Vitreous to resinous
Streak: Reddish
Transparency: Transparent

Is this ambrinoite?

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 ambrinoite with a known reference. Ambrinoite sits at Mohs <2, softer than the next harder reference, harder than the previous one.
2
Check the streak
Drag the specimen across an unglazed porcelain plate. Streak: reddish.
3
Read the luster
Hold the specimen under a strong light. Luster: vitreous to resinous.
4
Match the color range
Compare against the expected color range: cinnabar-red.
5
Look at form & habit
Crystal system: triclinic. Typical habit: prismatic crystals.

Often confused with

Ambrinoite vs. its common look-alikes, and how to tell them apart in the field.

mineral

Euxenite-(Y)

Mohs 5.5-6.5

How to tell apart: Euxenite-(Y) is the harder of the two (Mohs 5.5-6.5 vs. <2); streak differs, so Ambrinoite leaves reddish, Euxenite-(Y) leaves yellowish, grayish, or reddish brown; luster reads vitreous to resinous on Ambrinoite and brilliant submetallic, waxy to resinous on fractures on Euxenite-(Y).

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Often found alongside ambrinoite

Minerals reported to co-occur with ambrinoite. Spotting these in float or country rock is a strong cue you are in the right ground.

All properties

Chemical formula: [K,(NH₄)]₂(As,Sb)₆(Sb,As)₂S₁₃·H₂O
Mohs hardness: <2
Density: 3.276 g/cm³
Colors: Cinnabar-red
Streak: Reddish
Luster: Vitreous to resinous
Transparency: Transparent
Crystal system: Triclinic
Crystal habit: Prismatic crystals
Cleavage: Perfect on {001} and {010}, poor on {100}
Rarity: Rare
Uses: Collector
Host rock: In an evaporite deposit probably formed by highly alkaline, low-temperature hydrothermal fluids

Where rockhounds find ambrinoite

Classic worldwide localities

Cumbë Sûrdë quarry, Signols, Oulx, Upper Susa Valley, Torino, Piedmont, Italy

Field-hunting tip

Where it forms: In an evaporite deposit probably formed by highly alkaline, low-temperature hydrothermal fluids. If you start seeing sulfur, orpiment, gypsum in float, you are in the right ground. Typical habit: prismatic crystals. Train your eye for that shape before scanning the outcrop.

Common questions

How do you identify ambrinoite?+

Mohs hardness is <2. Luster: vitreous to resinous. The streak is reddish. Common colors include cinnabar-red.

Where is ambrinoite found?+

Notable localities include Cumbë Sûrdë quarry, Signols, Oulx, Upper Susa Valley, Torino, Piedmont, Italy.

Is ambrinoite safe to handle?+

Ambrinoite contains antimony and arsenic and calls for real care. Contains antimony and arsenic, which are toxic if ingested or inhaled as dust. Wash hands thoroughly after handling and avoid creating dust.

What rocks look like ambrinoite?+

Ambrinoite is most often confused with Euxenite-(Y). A quick hardness test and a streak check separate the look-alikes faster than color alone.

What minerals are found with ambrinoite?+

Ambrinoite commonly co-occurs with Sulfur, Orpiment, Gypsum. Spotting any of these in float or country rock is a useful trip signal.

What kind of rock does ambrinoite form in?+

Where it forms: In an evaporite deposit probably formed by highly alkaline, low-temperature hydrothermal fluids. Working float back to the host body is the standard way to chase a fresh occurrence.

What is ambrinoite used for?+

Ambrinoite is used in collector.

Find ambrinoite on the map

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