Willemite is world-renowned among collectors for its spectacular, brilliant green fluorescence under shortwave ultraviolet light. It is typically found in massive, granular, or prismatic forms, often intimately associated with franklinite and zincite in the unique deposits of New Jersey. Its name honors King William I of the Netherlands.
Is this willemite?
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 willemite with a known reference. Willemite sits at Mohs 5.5, softer than the next harder reference, harder than the previous one.
- 2Check the streakDrag the specimen across an unglazed porcelain plate. Streak: white.
- 3Read the lusterHold the specimen under a strong light. Luster: vitreous to resinous.
- 4Match the color rangeCompare against the expected color range: white, yellow, green, reddish brown, black.
- 5Look at form & habitCrystal system: trigonal. Typical habit: massive, prismatic.
Often confused with
Willemite vs. its common look-alikes, and how to tell them apart in the field.
Often found alongside willemite
Minerals reported to co-occur with willemite. Spotting these in float or country rock is a strong cue you are in the right ground.
All properties
- Chemical formula
- Zn₂SiO₄
- Mohs hardness
- 5.5
- Density
- 3.89-4.19 g/cm³
- Streak
- White
- Luster
- Vitreous to resinous
- Transparency
- Transparent to opaque
- Crystal system
- Trigonal
- Crystal habit
- Massive, prismatic
- Cleavage
- Indistinct on {0001}, {1120}
- Fluorescence
- Green under SW and LW UV
- Rarity
- Uncommon
- Uses
- Collector, Fluorescent mineral display, Ore of zinc
- Host rock
- Of secondary origin, especially in zinc deposits developed in limestones; while uncommon, locally an important ore of zinc
Where rockhounds find willemite
7 mapped spotsClassic worldwide localities
- Franklin and Sterling Hill, Ogdensburg, Sussex Co., New Jersey, USA
- Balmat, St. Lawrence Co., New York, USA
- Mammoth-St. Anthony mine, Tiger, Pinal Co., at the Red Cloud mine, La Paz Co., Arizona, USA
- Ygnacis and Cerro Gordo mines, Inyo Co., California, USA
- Altenberg, near Moresnet, Belgium
- Tsumeb and Guchab, Namibia
Field-hunting tip
Where it forms: Of secondary origin, especially in zinc deposits developed in limestones; while uncommon, locally an important ore of zinc. If you start seeing nasonite, glaucochroite, leucophoenicite in float, you are in the right ground. Typical habit: massive, prismatic. Train your eye for that shape before scanning the outcrop. In the U.S., the densest reported localities are in Arizona, New Jersey, New Mexico. Start trip planning there.
More willemite photos
Common questions
How do you identify willemite?+
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Can I find willemite in the United States?+
What rocks look like willemite?+
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Find willemite on the map
RockHoundR shows mapped rockhounding spots, access rules, and lets you log every find.
