Ilsemannite is a rare, water-soluble molybdenum oxide typically found as a bright blue or deep blue-black powdery coating on other minerals. It is known for its intense color, which often fades or changes upon exposure to dry air. Collectors usually keep it in sealed containers to prevent dehydration and loss of its distinctive blue coloration.
Is this ilsemannite?
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 ilsemannite with a known reference. Ilsemannite sits at Mohs 1-2 — softer than the next harder reference, harder than the previous one.
- 2Check the streakDrag the specimen across an unglazed porcelain plate. Ilsemannite leaves a blue-black streak.
- 3Read the lusterHold the specimen under a strong light. Ilsemannite typically shows a dull luster.
- 4Match the color rangeCompare against the expected color range: blue, black, dark gray.
- 5Look at form & habitCrystal system: amorphous. Typical habit: powdery coatings, efflorescent crusts.
Often confused with
Ilsemannite vs. its common look-alikes — and how to tell them apart in the field.
How to tell apart: Streak differs — Ilsemannite leaves blue-black, Molybdite leaves yellow; luster reads dull on Ilsemannite and silky on Molybdite.
How to tell apart: Streak differs — Ilsemannite leaves blue-black, Vivianite leaves white to light blue; luster reads dull on Ilsemannite and vitreous on Vivianite.
Often found alongside ilsemannite
Minerals reported to co-occur with ilsemannite. Spotting these in float or country rock is a strong cue you are in the right ground.
All properties
- Chemical formula
- Mo₃O₈·nH₂O
- Mohs hardness
- 1-2
- Density
- 2.8 g/cm³
- Streak
- Blue-black
- Luster
- Dull
- Transparency
- Opaque
- Crystal system
- Amorphous
- Crystal habit
- Powdery Coatings, Efflorescent Crusts
- Cleavage
- None
- Rarity
- Uncommon
- Uses
- Collector, Indicator Mineral
- Host rock
- Oxidized Zones of Molybdenum-bearing Ore Deposits
- Typical price
- $10-50 per specimen
Where rockhounds find ilsemannite
1 mapped spotsClassic worldwide localities
- Bleiberg, Austria
- Climax Mine, Colorado, USA
- Questa Mine, New Mexico, USA
- Telluride, Colorado, USA
Field-hunting tip
Look in oxidized zones of molybdenum-bearing ore deposits country — that is the host setting where ilsemannite typically forms. If you start seeing molybdenite, limonite, gypsum in float, you are in the right ground. Field specimens usually show a powdery coatings, efflorescent crusts habit, so train your eye for that shape before scanning the outcrop. In the U.S., the densest reported localities are in Wyoming — start trip planning there.
Common questions
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Find ilsemannite on the map
RockHoundR shows mapped rockhounding spots, access rules, and lets you log every find.
