Millerite Identification Guide
How to identify Millerite, a brass-yellow nickel sulfide, by its fine radiating needle crystals, metallic luster, and greenish-black streak.
Read the full Millerite encyclopedia entry →
What Millerite Looks Like
Millerite is a nickel sulfide (NiS) famous for forming slender, hair-like to needle-like crystals, often as radiating sprays or tangled "wire" tufts inside cavities and geodes. It has a striking pale brass-yellow color with a bright metallic shine.
- Color: Pale brass-yellow to bronze-yellow, sometimes with iridescent gray tarnish.
- Luster: Metallic.
- Transparency: Opaque.
- Habit/form: Very fine acicular (needle) crystals, hair-like sprays, radiating clusters, velvety coatings.
Step-by-Step Field ID Checklist
- Look for the needles: Diagnostic thin, brassy, often curved hair-like crystals radiating from a point.
- Note the color and shine: Bright pale-brass metallic, distinctly different from silvery or red-brown ores.
- Check the setting: Commonly found lining vugs in limestone, dolomite, chert geodes, or in nickel ore veins.
- Test streak (below): Greenish-black streak helps confirm.
- Confirm hardness with a knife (below).
Key Diagnostic Tests
- Mohs hardness: ~3–3.5 — soft; a knife scratches it.
- Streak: Greenish-black to black.
- Cleavage: Perfect rhombohedral cleavage (hard to see in fine needles); needles are brittle.
- Density: High, ~5.3–5.65 g/cm3 (heavy for its size).
- Acid: Soluble in nitric acid (gives a green nickel solution); not a field-casual test.
- Magnetism: Non-magnetic.
Common Look-Alikes and How to Tell Them Apart
- Pyrite / marcasite: Harder (6–6.5), form cubes or blades rather than fine hairs; pyrite is paler-gold and cannot be scratched by a knife.
- Chalcopyrite: Brassier-golden, massive, harder (~3.5–4) but does not form fine radiating needles; greenish-black streak too, so use habit (needles) to separate.
- Gold: Soft but malleable (dents, does not powder), much heavier, yellow streak.
- Rutile / goethite needles: Reddish to brown-black, not brassy metallic, and non-sulfide.
- Bismuthinite / stibnite: More silvery-gray bladed crystals, not brass-yellow.
Where It Is Found
Millerite occurs in nickel-bearing hydrothermal veins and as a cavity mineral in carbonate rocks and geodes. Classic localities include Wales (UK), the Sudbury district (Canada), Germany, and famous chert geodes in the U.S. (Iowa, Wisconsin, Missouri).
Frequently asked questions
How can you tell if it is millerite?
Look for fine, brass-yellow, hair-like or needle crystals radiating in sprays, often inside geode or vug cavities. It is soft (knife scratches it, hardness 3–3.5), heavy, metallic, and leaves a greenish-black streak.
What does millerite look like?
It looks like tufts of thin, brassy-gold metallic needles or hairs radiating from a center, often clustered inside cavities and on matrix.
Millerite vs pyrite — how do you tell them apart?
Pyrite is harder (6–6.5, a knife will not scratch it) and forms cubes or massive grains, while millerite is soft (3–3.5) and forms distinctive fine brassy needle sprays.
Is millerite valuable or is it a nickel ore?
Millerite is a nickel sulfide and a minor nickel ore, but well-formed needle sprays in geodes are prized mainly as collector specimens rather than as a mining ore.