Cossaite is a historical term used for a massive, milky-white variety of quartz. It typically occurs as dense, non-crystalline veins or masses and is often characterized by its opaque, porcelain-like appearance.

Hardness

Mohs

Luster

Vitreous

Streak

White

Transparency

Opaque

mineralcommonQuartz family

Cossaite

Hardness: 7
Luster: Vitreous
Streak: White
Transparency: Opaque

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Is this cossaite?

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 cossaite with a known reference. Cossaite sits at Mohs 7 — softer than the next harder reference, harder than the previous one.
2
Check the streak
Drag the specimen across an unglazed porcelain plate. Cossaite leaves a white streak.
3
Read the luster
Hold the specimen under a strong light. Cossaite typically shows a vitreous luster.
4
Match the color range
Compare against the expected color range: white, gray.
5
Look at form & habit
Crystal system: trigonal. Typical habit: massive.

Often confused with

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

How to tell apart: Luster reads vitreous on Cossaite and waxy on Chalcedony.

How to tell apart: Luster reads vitreous on Cossaite and waxy on Flint Nodules.

Compare →

Often found alongside cossaite

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

All properties

Chemical formula: SiO₂
Mohs hardness: 7
Density: 2.65 g/cm³
Colors: White Gray
Streak: White
Luster: Vitreous
Transparency: Opaque
Crystal system: Trigonal
Crystal habit: Massive
Cleavage: None
Rarity: Common
Uses: Collector, Lapidary
Host rock: Hydrothermal Veins
Typical price: $5-20

Where rockhounds find cossaite

Classic worldwide localities

Italy
Switzerland
United States

Field-hunting tip

Look in hydrothermal veins country — that is the host setting where cossaite typically forms. If you start seeing quartz, calcite, dolomite in float, you are in the right ground. Field specimens usually show a massive habit, so train your eye for that shape before scanning the outcrop.

Common questions

How do you identify cossaite?+

Mohs hardness is 7. It typically shows a vitreous luster. The streak is white. Common colors include white, gray.

Where is cossaite found?+

Notable localities include Italy; Switzerland; United States.

How much is cossaite worth?+

Typical asking prices fall in the range of $5-20. Quality, size, and provenance can move individual specimens well outside that range.

What rocks look like cossaite?+

Cossaite is most often confused with Quartz, Chalcedony, Flint Nodules. A quick hardness test and a streak check separate the look-alikes faster than color alone.

What minerals are found with cossaite?+

Cossaite commonly co-occurs with Quartz, Calcite, Dolomite. Spotting any of these in float or country rock is a useful trip signal.

What kind of rock does cossaite form in?+

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

What is cossaite used for?+

Cossaite is used in collector, lapidary.

Find cossaite on the map

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