If you think it's gross that there are beetles in your nail polish, you're not going to like this truth about lipstick.
When scales, which have been used to create that same pearl-like look in jewelry, are layered on top of one another, they have a crystallized effect. Now, thanks to Gawker's science-focused offshoot io9, we know why:
As light hits the plane of each crystal, some light is reflected back, and some keeps going down until it hits another crystal. As light gets thrown up, in a diffuse way, from many different levels, the scales seem to emanate light from within. Slather enough layers on a bead, and it has a glowing, pearlescent quality. If it works on a bead, it must do the same for lipstick.
How can you tell if your makeup contains it? Check the ingredients list for guanine, the name by which this crystalline substance made from fish scales is typically listed.
And if slathering on fish scales just isn't your thing, here is a list of PETA-approved lipsticks for you to try, instead.