The word mica is thought to be derived from the Latin word micare, which means "to glitter," and as it relates to soap making, mica certainly does!
Mica and Its Uses
Mica is a general name for a group of natural Earth minerals that are mined, purified and pulverized into fine powders and then used for dozens of different applications including cosmetics and soap making. The minerals that mica comes from having a fragile, layered crystalline structure gives it a pearlescent and metallic look. Mica is what gives the metallic paint finish on a car it's depth and glitter.
Much of the mica we use in soap making is even more complex than just a powdered mineral powder. Much of the mica powders we use are coated with either iron oxides or titanium dioxide, which gives them an even more multi-layered, complex color. That’s why when you look at the list of ingredients, it will often list several components.
Since it relies on reflected light, mica works best in transparent soaps like melt and pour. About 1/2 tsp. per pound of soap is a good place to start. This will give you a nice pearlescent effect. Experiment; combine it with other colorants and use just a bit of mica in a swirl, but not the whole bar. Brush the mica into the mold similar to how one might melt and pour gemstone soaps.
If melt and pour soap isn't your thing, some micas will work in cold process soap - but some will not. Their colors are sometimes unstable in the high ph of raw cold processed soap. Do a small test batch first or test out a little bit of mica in a little bit of lye water to see if the color shifts.