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, 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.