The cosmetic ingredient derived from fish scales and used in nail polish is guanine.
Guanine is a nitrogen-containing compound found naturally in DNA and RNA, but cosmetics use a crystalline form obtained from fish scales. Its plate-like crystals reflect and scatter light, producing a pearly, iridescent shimmer in nail polish, eye shadow, and other products. The material is sometimes known as pearl essence.
In 1656, a Parisian named Jaquin extracted crystalline guanine from the scales of Alburnus alburnus, the common bleak. This discovery helped establish fish-scale material as a source of decorative luster long before modern synthetic pearlescent pigments became widespread.
Guanine is easy to confuse with mica or titanium dioxide, which can also create shimmer or opacity in cosmetics. Carmine is different again: it is a red pigment made from cochineal insects. Guanine’s distinctive role is its reflective, pearl-like appearance rather than its color.