Specimen of the minerals willemite and calcite fluorescing under ultraviolet light. Willemite is zinc silicate (Zn2SiO4), and fluoresces green. Calcite is calcium carbonate (CaCO3) and fluoresces red. Fluorescence is a property of a material in which it absorbs light at one wavelength, and re-emits it at a longer wavelength, often absorbing invisible UV and emitting visible light. This specimen is from Franklin Mine, New Jersey, USA, a unique location noted for its rare and fluorescent minerals.