Crookesite is an exceptionally rare copper-thallium-silver selenide mineral typically found as small, lead-gray metallic grains in hydrothermal deposits. Collectors prize it primarily for its historical significance in the discovery of the element thallium and its inclusion in rare-earth selenide assemblages. It is almost exclusively found in association with other selenide minerals and requires careful handling due to its toxic chemical composition.
Is this crookesite?
5-step field checkRun through these checks against the specimen in your hand. The more boxes tick, the more confident the ID.
- 1Test the hardnessTry to scratch crookesite with a known reference. Crookesite sits at Mohs 2.5-3 — softer than the next harder reference, harder than the previous one.
- 2Check the streakDrag the specimen across an unglazed porcelain plate. Crookesite leaves a lead-gray streak.
- 3Read the lusterHold the specimen under a strong light. Crookesite typically shows a metallic luster.
- 4Match the color rangeCompare against the expected color range: lead-gray, bluish-white.
- 5Look at form & habitCrystal system: tetragonal. Typical habit: massive, granular, or compact grains.
Often confused with
Crookesite vs. its common look-alikes — and how to tell them apart in the field.
How to tell apart: Streak differs — Crookesite leaves lead-gray, Eucairite leaves black.
How to tell apart: Streak differs — Crookesite leaves lead-gray, Berzelianite leaves black.
How to tell apart: Streak differs — Crookesite leaves lead-gray, Clausthalite leaves gray-black.
Often found alongside crookesite
Minerals reported to co-occur with crookesite. Spotting these in float or country rock is a strong cue you are in the right ground.
All properties
- Chemical formula
- (Cu,Tl,Ag)₂Se
- Mohs hardness
- 2.5-3
- Density
- 6.9 g/cm³
- Colors
- Streak
- Lead-gray
- Luster
- Metallic
- Transparency
- Opaque
- Crystal system
- Tetragonal
- Crystal habit
- Massive, Granular, Or Compact Grains
- Cleavage
- None
- Rarity
- Rare
- Uses
- Collector, Scientific Research
- Host rock
- Hydrothermal Selenide Deposits
- Typical price
- $50-300 per small specimen
Where rockhounds find crookesite
Classic worldwide localities
- Skrikerum, Sweden
- Tumiñico, Argentina
- Siekierki, Poland
Field-hunting tip
Look in hydrothermal selenide deposits country — that is the host setting where crookesite typically forms. If you start seeing eucairite, berzelianite, clausthalite in float, you are in the right ground. Field specimens usually show a massive, granular, or compact grains habit, so train your eye for that shape before scanning the outcrop.
Common questions
How do you identify crookesite?+
Where is crookesite found?+
How much is crookesite worth?+
Is crookesite safe to handle?+
What rocks look like crookesite?+
What minerals are found with crookesite?+
What kind of rock does crookesite form in?+
What is crookesite used for?+
Find crookesite on the map
RockHoundR shows mapped rockhounding spots, access rules, and lets you log every find.
