Williamsite is a translucent, apple-green variety of antigorite serpentine often containing small black inclusions of chromite or magnetite. It is prized by lapidary enthusiasts for its attractive color and ease of carving, often polished into cabochons. It is primarily found in serpentinized ultramafic rocks and is famously associated with the deposits in the State Line serpentine barrens of Pennsylvania and Maryland.
Is this williamsite?
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 williamsite with a known reference. Williamsite sits at Mohs 2.5-3.5 — softer than the next harder reference, harder than the previous one.
- 2Check the streakDrag the specimen across an unglazed porcelain plate. Williamsite leaves a white streak.
- 3Read the lusterHold the specimen under a strong light. Williamsite typically shows a waxy luster.
- 4Match the color rangeCompare against the expected color range: apple-green, olive-green, yellowish-green.
- 5Look at form & habitCrystal system: monoclinic. Typical habit: massive.
Often confused with
Williamsite vs. its common look-alikes — and how to tell them apart in the field.
How to tell apart: Nephrite Jade is the harder of the two (Mohs 6-6.5 vs. 2.5-3.5); luster reads waxy on Williamsite and greasy on Nephrite Jade.
How to tell apart: Prehnite is the harder of the two (Mohs 6-6.5 vs. 2.5-3.5); luster reads waxy on Williamsite and vitreous on Prehnite.
How to tell apart: Oregon Jade is the harder of the two (Mohs 6.5-7 vs. 2.5-3.5).
Often found alongside williamsite
Minerals reported to co-occur with williamsite. Spotting these in float or country rock is a strong cue you are in the right ground.
All properties
- Chemical formula
- (Mg,Fe)₃Si₂O₅(OH)₄
- Mohs hardness
- 2.5-3.5
- Density
- 2.5-2.6 g/cm³
- Streak
- White
- Luster
- Waxy
- Transparency
- Translucent
- Crystal system
- Monoclinic
- Crystal habit
- Massive
- Cleavage
- None
- Rarity
- Uncommon
- Uses
- Lapidary, Collector, Decorative
- Host rock
- Serpentinite
- Typical price
- $10-100 per specimen
Where rockhounds find williamsite
5 mapped spotsClassic worldwide localities
- Pennsylvania, USA
- Maryland, USA
- New Zealand
- Afghanistan
Field-hunting tip
Look in serpentinite country — that is the host setting where williamsite typically forms. If you start seeing chromite, magnetite, calcite in float, you are in the right ground. Field specimens usually show a massive habit, so train your eye for that shape before scanning the outcrop. In the U.S., the densest reported localities are in Pennsylvania, Maryland — start trip planning there.
Common questions
How do you identify williamsite?+
Where is williamsite found?+
Can I find williamsite in the United States?+
How much is williamsite worth?+
What rocks look like williamsite?+
What minerals are found with williamsite?+
What kind of rock does williamsite form in?+
What is williamsite used for?+
Find williamsite on the map
RockHoundR shows mapped rockhounding spots, access rules, and lets you log every find.
