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, 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.