Ludwigite is a magnesium-iron borate typically found in contact metamorphic rocks, specifically skarns formed from limestone intruded by igneous bodies. It is most easily identified by its characteristic black to dark-green fibrous or acicular crystal habit often occurring in radiating sprays.
Is this ludwigite?
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 ludwigite with a known reference. Ludwigite sits at Mohs 5 — softer than the next harder reference, harder than the previous one.
- 2Check the streakDrag the specimen across an unglazed porcelain plate. Ludwigite leaves a black streak.
- 3Read the lusterHold the specimen under a strong light. Ludwigite typically shows a submetallic luster.
- 4Match the color rangeCompare against the expected color range: black, dark green, dark brown.
- 5Look at form & habitCrystal system: orthorhombic. Typical habit: fibrous, acicular, prismatic crystals, radiating aggregates.
Often confused with
Ludwigite vs. its common look-alikes — and how to tell them apart in the field.
How to tell apart: Tourmaline is the harder of the two (Mohs 7-7.5 vs. 5); streak differs — Ludwigite leaves black, Tourmaline leaves white; luster reads submetallic on Ludwigite and vitreous on Tourmaline.
How to tell apart: Aegirine is the harder of the two (Mohs 6 vs. 5); streak differs — Ludwigite leaves black, Aegirine leaves yellowish-grey; luster reads submetallic on Ludwigite and vitreous on Aegirine.
Often found alongside ludwigite
Minerals reported to co-occur with ludwigite. Spotting these in float or country rock is a strong cue you are in the right ground.
All properties
- Chemical formula
- Mg₂Fe³⁺(BO₃)O₂
- Mohs hardness
- 5
- Density
- 3.9-4.7 g/cm³
- Streak
- Black
- Luster
- Submetallic
- Transparency
- Opaque
- Crystal system
- Orthorhombic
- Crystal habit
- Fibrous, Acicular, Prismatic Crystals, Radiating Aggregates
- Cleavage
- None
- Rarity
- Uncommon
- Uses
- Collector, Scientific Research
- Host rock
- Skarn Deposits in Contact-metamorphosed Limestone
- Typical price
- $15-80 for thumbnail to small cabinet specimens
Where rockhounds find ludwigite
1 mapped spotsClassic worldwide localities
- Băița, Romania
- Dashkesan, Azerbaijan
- Riverside County, California, USA
- Kaveltorp, Sweden
- Suan, North Korea
Field-hunting tip
Look in skarn deposits in contact-metamorphosed limestone country — that is the host setting where ludwigite typically forms. If you start seeing magnetite, forsterite, calcite in float, you are in the right ground. Field specimens usually show a fibrous, acicular, prismatic crystals, radiating aggregates habit, so train your eye for that shape before scanning the outcrop. In the U.S., the densest reported localities are in Utah — start trip planning there.
Common questions
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Find ludwigite on the map
RockHoundR shows mapped rockhounding spots, access rules, and lets you log every find.
