Tridymite is a high-temperature polymorph of silica found primarily in acidic volcanic rocks like rhyolite and andesite. It typically occurs as small, thin, tabular, pseudo-hexagonal crystals often found within cavities or vesicles in volcanic matrices.

Hardness

6.5

Mohs

Luster

Vitreous

Streak

White

Transparency

Transparent

mineraluncommonSilica Mineral family

Tridymite

Hardness: 6.5
Luster: Vitreous
Streak: White
Transparency: Transparent

Is this tridymite?

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 tridymite with a known reference. Tridymite sits at Mohs 6.5 — softer than the next harder reference, harder than the previous one.
2
Check the streak
Drag the specimen across an unglazed porcelain plate. Tridymite leaves a white streak.
3
Read the luster
Hold the specimen under a strong light. Tridymite typically shows a vitreous luster.
4
Match the color range
Compare against the expected color range: colorless, white, gray.
5
Look at form & habit
Crystal system: hexagonal. Typical habit: tabular crystals, wedge-shaped twins, massive.

Often confused with

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

Often found alongside tridymite

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

All properties

Chemical formula: SiO₂
Mohs hardness: 6.5
Density: 2.26 g/cm³
Colors: Colorless White Gray
Streak: White
Luster: Vitreous
Transparency: Transparent
Crystal system: Hexagonal
Crystal habit: Tabular Crystals, Wedge-shaped Twins, Massive
Cleavage: Poor
Rarity: Uncommon
Uses: Collector, Scientific Research
Host rock: Rhyolite, Andesite
Typical price: $10-50 thumbnail, $50-200 cabinet

Where rockhounds find tridymite

Classic worldwide localities

Mexico
Germany
Japan
USA

Field-hunting tip

Look in rhyolite, andesite country — that is the host setting where tridymite typically forms. If you start seeing sanidine, fayalite, magnetite in float, you are in the right ground. Field specimens usually show a tabular crystals, wedge-shaped twins, massive habit, so train your eye for that shape before scanning the outcrop.

Common questions

How do you identify tridymite?+

Mohs hardness is 6.5. It typically shows a vitreous luster. The streak is white. Common colors include colorless, white, gray.

Where is tridymite found?+

Notable localities include Mexico; Germany; Japan; USA.

How much is tridymite worth?+

Typical asking prices fall in the range of $10-50 thumbnail, $50-200 cabinet. Quality, size, and provenance can move individual specimens well outside that range.

What rocks look like tridymite?+

Tridymite is most often confused with Quartz, Cristobalite, Opal. A quick hardness test and a streak check separate the look-alikes faster than color alone.

What minerals are found with tridymite?+

Tridymite commonly co-occurs with Sanidine, Fayalite, Magnetite, Hematite. Spotting any of these in float or country rock is a useful trip signal.

What kind of rock does tridymite form in?+

Tridymite typically forms in rhyolite, andesite. Working float back to the host body is the standard way to chase a fresh occurrence.

What is tridymite used for?+

Tridymite is used in collector, scientific research.

Find tridymite on the map

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