Although massive willemite is much more common, some crystals do show the rhombohedral terminations atop hexagonal prisms that is characteristic of it symmetry. Willemite's great desirability is not so much its use as a zinc ore, its rarity, its associations or even its crystal habits as it is its wonderful fluorescence. Nearly all willemite specimens fluoresce a bright green under ultra-violet light. This fluorescence combined with the red fluorescence of the calcite at Franklin, makes specimens from there even more special. Some willemite specimens will even show phosphorescence.
Phosphorescence is the ability of a mineral to glow after the initial light is removed.
The mineral has essentially stored the energy of the initial activating light and re-emits light on a delayed basis. Willemite is one of the best examples of a fluorescent mineral and is a must have for any collector interested in this phenomenon. Color is usually colorless or white but can be tinted yellow, blue, red, brown and often green. Luster is vitreous to resinous. Transparency crystals are transparent to translucent.