Ondrušite is a rare hydrated calcium copper arsenate mineral found in the hydrothermal veins of the Jáchymov district. It typically occurs as small tabular crystals or crusts that form in association with other arsenic-bearing secondary minerals.

Hardness

Mohs

Luster

Vitreous

Streak

White

Transparency

Transparent

mineralrareArmenite Group family

Ondrušite

Hardness: 4
Luster: Vitreous
Streak: White
Transparency: Transparent

Is this ondrušite?

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 ondrušite with a known reference. Ondrušite sits at Mohs 4 — softer than the next harder reference, harder than the previous one.
2
Check the streak
Drag the specimen across an unglazed porcelain plate. Ondrušite leaves a white streak.
3
Read the luster
Hold the specimen under a strong light. Ondrušite typically shows a vitreous luster.
4
Match the color range
Compare against the expected color range: white, colorless.
5
Look at form & habit
Crystal system: orthorhombic. Typical habit: tabular crystals, aggregates.

Often confused with

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

mineral

Pharmacolite

Mohs 2-2.5

How to tell apart: Ondrušite is noticeably harder (Mohs 4 vs. 2-2.5).

How to tell apart: Ondrušite is noticeably harder (Mohs 4 vs. 2-2.5).

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Often found alongside ondrušite

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

All properties

Chemical formula: CaCu₆(AsO₄)₂(AsO₃OH)₂·10H₂O
Mohs hardness: 4
Density: 3.1-3.2 g/cm³
Colors: White Colorless
Streak: White
Luster: Vitreous
Transparency: Transparent
Crystal system: Orthorhombic
Crystal habit: Tabular Crystals, Aggregates
Cleavage: Distinct
Rarity: Rare
Uses: Collector
Host rock: Hydrothermal Veins
Typical price: $50-300 per specimen

Where rockhounds find ondrušite

Classic worldwide localities

Jáchymov, Czech Republic

Field-hunting tip

Look in hydrothermal veins country — that is the host setting where ondrušite typically forms. If you start seeing arsenolite, picropharmacolite, guerinite in float, you are in the right ground. Field specimens usually show a tabular crystals, aggregates habit, so train your eye for that shape before scanning the outcrop.

Common questions

How do you identify ondrušite?+

Mohs hardness is 4. It typically shows a vitreous luster. The streak is white. Common colors include white, colorless.

Where is ondrušite found?+

Notable localities include Jáchymov, Czech Republic.

How much is ondrušite 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 ondrušite safe to handle?+

It contains toxic constituents. Contains arsenic, which is toxic if ingested or inhaled as dust. Always wash hands after handling and keep away from children or pets. Handle with care, avoid grinding or breathing dust, and store separately.

What rocks look like ondrušite?+

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

What minerals are found with ondrušite?+

Ondrušite commonly co-occurs with Arsenolite, Picropharmacolite, Guerinite. Spotting any of these in float or country rock is a useful trip signal.

What kind of rock does ondrušite form in?+

Ondrušite typically forms in hydrothermal veins. Working float back to the host body is the standard way to chase a fresh occurrence.

What is ondrušite used for?+

Ondrušite is used in collector.

Find ondrušite on the map

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