Fetiasite is an extremely rare arsenic-bearing mineral known primarily from the type locality in the Aosta Valley, Italy. It typically forms small, dark, submetallic tabular crystals embedded within manganese-rich metamorphic rocks.
Is this fetiasite?
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 fetiasite with a known reference. Fetiasite sits at Mohs 3-4 — softer than the next harder reference, harder than the previous one.
- 2Check the streakDrag the specimen across an unglazed porcelain plate. Fetiasite leaves a yellowish brown streak.
- 3Read the lusterHold the specimen under a strong light. Fetiasite typically shows a submetallic luster.
- 4Match the color rangeCompare against the expected color range: dark brown, black.
- 5Look at form & habitCrystal system: monoclinic. Typical habit: tabular crystals.
Often confused with
Fetiasite vs. its common look-alikes — and how to tell them apart in the field.
How to tell apart: Manaccanite is the harder of the two (Mohs 5-6 vs. 3-4); streak differs — Fetiasite leaves yellowish brown, Manaccanite leaves black.
How to tell apart: Iron Ore is the harder of the two (Mohs 5-6.5 vs. 3-4); streak differs — Fetiasite leaves yellowish brown, Iron Ore leaves reddish-brown to black; luster reads submetallic on Fetiasite and metallic to submetallic on Iron Ore.
Often found alongside fetiasite
Minerals reported to co-occur with fetiasite. Spotting these in float or country rock is a strong cue you are in the right ground.
All properties
- Chemical formula
- (Fe²⁺,Fe³⁺,Ti)₃(As³⁺O₃)₄
- Mohs hardness
- 3-4
- Density
- 4.2 g/cm³
- Colors
- Streak
- Yellowish Brown
- Luster
- Submetallic
- Transparency
- Opaque
- Crystal system
- Monoclinic
- Crystal habit
- Tabular Crystals
- Cleavage
- None Observed
- Rarity
- Rare
- Uses
- Collector
- Host rock
- Metamorphosed Manganese Deposits
- Typical price
- $50-300 per specimen
Where rockhounds find fetiasite
Classic worldwide localities
- Fetias, Saint-Marcel, Aosta Valley, Italy
Field-hunting tip
Look in metamorphosed manganese deposits country — that is the host setting where fetiasite typically forms. If you start seeing braunite, quartz, hematite in float, you are in the right ground. Field specimens usually show a tabular crystals habit, so train your eye for that shape before scanning the outcrop.
Common questions
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Find fetiasite on the map
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