Willemite Identification Guide

What Willemite Looks Like

Willemite is zinc silicate (Zn2SiO4). In ordinary light it is variable — greenish, yellow-green, brown, reddish, gray, or white — with a vitreous to resinous luster and transparent to translucent clarity. It forms hexagonal (trigonal) prismatic crystals, but is most often massive or granular. Its single most famous property is its brilliant green fluorescence under shortwave ultraviolet light, which makes the Franklin, New Jersey ores glow vividly.

Key Visual Cues

• Variable body color (green, yellow, brown, red, white)
• Vitreous to resinous luster
• Massive/granular or stubby hexagonal prisms
• Intense green glow under shortwave UV

Step-by-Step Field ID Checklist

1. Test UV fluorescence. Shine a shortwave UV lamp — bright green fluorescence is the classic willemite signature.
2. Check the association. At Franklin/Sterling Hill it occurs with red fluorescent calcite and black franklinite.
3. Test hardness. It scratches glass faintly (Mohs 5.5) but is softer than quartz.
4. Heft it. Zinc content makes it moderately dense (SG ~3.9–4.2).
5. Examine luster and color. Resinous luster and green-to-brown tones support the ID.

Diagnostic Tests

• Mohs hardness: 5.5 — scratches glass weakly; quartz scratches it.
• Streak: White.
• Cleavage/fracture: Poor to distinct cleavage; conchoidal to uneven fracture.
• Density: ~3.9–4.2 g/cm³ — heavy for a silicate.
• Fluorescence: Bright green under shortwave UV (and often phosphorescent) — diagnostic.

Common Look-Alikes and How to Tell Them Apart

• Other zinc minerals (hemimorphite, smithsonite): Differ in habit and reactivity — smithsonite is a carbonate that fizzes in acid; willemite (a silicate) does not, and willemite's green UV glow is distinctive.
• Olivine / green silicates: Lack willemite's strong green UV fluorescence and high density.
• Apatite: Can fluoresce but is a phosphate with different habit; willemite's Franklin association and green glow distinguish it.
• Prehnite / epidote: Greenish but non-fluorescent and lower density.
• Franklinite (associated): Black, magnetic oxide — easily separated from willemite by color and magnetism.

Where Willemite Is Found

Willemite is most famous from the zinc deposits of Franklin and Sterling Hill, New Jersey, where it is a major ore mineral alongside franklinite and zincite. It also occurs as a secondary mineral in oxidized zinc deposits worldwide, including Belgium (the type locality is named for King William I), Namibia (Tsumeb), Greenland, and the southwestern United States.

Collecting Tips

Carry a shortwave UV lamp — willemite is far easier to spot by its green glow than by daylight color. In Franklin-type ore, look for the dramatic combination of green-fluorescing willemite and red-fluorescing calcite. Confirm with hardness around 5.5 and a notably high density.