Fish scales have a certain pearly quality to them.

That glow makes them a common ingredient in various shimmery makeup, lipstick included.

How can you tell if your lipstick contains it?

Check the ingredients list for guanine.

It is the name by which this crystalline substance made from fish scales is typically listed.

The other ingredient is also called pearl essence.

It is primarily sourced from herring and can be found in many lipsticks, nail polishes and other cosmetic products due to the shimmer effect it creates. 

Pearl essence is one of many by-products of large-scale commercial fish processing.