Mica is a common main ingredient in mineral eyeshadows. The term actually refers to a group of silicate minerals with very uniform cleavage (that's how it breaks, you guys) caused by the uniform hexagonal structure. This makes it useful for mineral cosmetics because it forms uniformly-sized granules which usually appear glittery or shimmery, like sand, but without a scratchy or gritty texture. The amount of sparkle increases with the increased size of the particles -- conversely, low-sheen or matte micas have a smaller particle size. The opacity of mica powder decreases with the increased size of the particles, so sparkly micas will tend to be sheer. The opacity can also be altered using metal oxides, particularly titanium dioxide, which is very white and opaque. Micas are also used in mineral foundations and finishing powders and veils, although the latter are often made with lighter mineral powders such as silica and talc.
Micas are composed of various metals bonded with silicates (silicon+oxygen), oxygen, hydroxides (OH, oxygen+hydrogen), and sometimes flourides. For example, one kind of mica is called aluminoceladonite, or potassium aluminum magnesium iron silicate hydroxide. There are many unique chemical structures that fall in this category, so it's much easier to just call them micas. They are defined by their tetrahedral structure and form layers or sheets which can be sheered or flaked into large flat pieces. Thin, translucent sheets of mica are known as isinglass. Because micas have stable structures, the material is not reactive and is useful in many applications such as electronics. This chemical stability also makes the ingredient non-irritating for cosmetic use. Most allergic reactions to mineral cosmetics are caused by another mineral, bismuth oxychloride, which is losing popularity as a cosmetic ingredient for that reason.