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 have a fragile, layered crystalline structure gives it a pearlescent and/or metallic look. Mica is what gives the metallic paint finish on a car it's depth and glitter.
And much of the mica we use in soap making is actually 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.
Because it relies on reflected light, mica works best in transparent soaps like melt and pour. About 1/2 tsp. per pound of soap is good place to start. This will give you a nice pearlescent effect. But experiment - combine it with other colorants...use just a bit of mica in a swirl, but not the whole bar. Brush the mica into the mold like I do with my melt and pour gemstone soaps. Note: if the melt and pour base is too hot (too thin) the mica will sink to the bottom. Be sure to keep stirring until the soap is just starting to thicken up.
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. I've done lovely gold and copper veins & swirls in Christmas themed soaps. Have fun and try out some mica in your soap!
Examples of micas (and other colorants) in soap swirls: