Gerstleyite is an extremely rare sulfosalt mineral found primarily in borate deposits in California. It typically occurs as reddish-orange crusts or small fibrous aggregates associated with colemanite and realgar.

Hardness

1.5-2

Mohs

Luster

Resinous

Streak

Orange

Transparency

Translucent

mineralrare

Gerstleyite

Hardness: 1.5-2
Luster: Resinous
Streak: Orange
Transparency: Translucent

Is this gerstleyite?

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 gerstleyite with a known reference. Gerstleyite sits at Mohs 1.5-2 — softer than the next harder reference, harder than the previous one.
2
Check the streak
Drag the specimen across an unglazed porcelain plate. Gerstleyite leaves a orange streak.
3
Read the luster
Hold the specimen under a strong light. Gerstleyite typically shows a resinous luster.
4
Match the color range
Compare against the expected color range: red, orange, brown.
5
Look at form & habit
Crystal system: monoclinic. Typical habit: massive, crusts, or radiating fibrous aggregates.

Often confused with

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

mineral

Realgar

Mohs 1.5-2

How to tell apart: Streak differs — Gerstleyite leaves orange, Realgar leaves orange-red.

How to tell apart: Streak differs — Gerstleyite leaves orange, Cinnabar leaves scarlet; luster reads resinous on Gerstleyite and adamantine on Cinnabar.

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Often found alongside gerstleyite

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

All properties

Chemical formula: Na₂HgSb₈S₁₆·2H₂O
Mohs hardness: 1.5-2
Density: 3.5 g/cm³
Colors: Red Orange Brown
Streak: Orange
Luster: Resinous
Transparency: Translucent
Crystal system: Monoclinic
Crystal habit: Massive, Crusts, Or Radiating Fibrous Aggregates
Cleavage: Perfect in One Direction
Rarity: Rare
Uses: Collector
Host rock: Evaporite Deposits in Borate Mines
Typical price: $50-300 per specimen

Where rockhounds find gerstleyite

Classic worldwide localities

Boron, California, USA

Field-hunting tip

Look in evaporite deposits in borate mines country — that is the host setting where gerstleyite typically forms. If you start seeing colemanite, ulexite, realgar in float, you are in the right ground. Field specimens usually show a massive, crusts, or radiating fibrous aggregates habit, so train your eye for that shape before scanning the outcrop.

Common questions

How do you identify gerstleyite?+

Mohs hardness is 1.5-2. It typically shows a resinous luster. The streak is orange. Common colors include red, orange, brown.

Where is gerstleyite found?+

Notable localities include Boron, California, USA.

How much is gerstleyite worth?+

Typical asking prices fall in the range of $50-300 per specimen. Quality, size, and provenance can move individual specimens well outside that range.

Is gerstleyite safe to handle?+

It contains toxic constituents. Contains mercury and arsenic. Wear gloves when handling and avoid inhaling dust. Wash hands thoroughly after contact. Handle with care, avoid grinding or breathing dust, and store separately.

What rocks look like gerstleyite?+

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

What minerals are found with gerstleyite?+

Gerstleyite commonly co-occurs with Colemanite, Ulexite, Realgar, Stibnite. Spotting any of these in float or country rock is a useful trip signal.

What kind of rock does gerstleyite form in?+

Gerstleyite typically forms in evaporite deposits in borate mines. Working float back to the host body is the standard way to chase a fresh occurrence.

What is gerstleyite used for?+

Gerstleyite is used in collector.

Find gerstleyite on the map

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