The formula is described as a "finely milled mixture of mica and pigments" that have a "subtle glaze of colour" but have an "intense pearl" finish; they even recommend to apply it "sparingly at first" because it can be quite concentrated. It can be used on "bare skin," "mixed into" foundation or moisturizer, or applied "over foundation." The consistency was finely-milled, silky, and very soft to the touch. As it is a loose powder, it can be quite pigmented or sheer depending on how much is picked up on a brush; I felt like a moderate pickup of color (coating the edges of a tapered highlighting brush) yielded good color coverage in a single layer that diffused easily on my skin. There was a moderate sheen on my skin once the product was blended out, and there was subtle to moderate color visible depending on the shade. The formula lasted eight hours on me before fading noticeably.
It applied well over bare skin or over foundation, and if one wanted to mix it into foundation or moisturizer, that was certainly doable, but you'll want to shake out a good bit of product and mix in the palm of your hand or on the back of your hand to thoroughly disperse the product.
The average rating based on all reviewed shade(s) in this formula is shown below.
M·A·C Iridescent Powder/Loose is a finely milled mixture of mica and pigments that gives skin a subtle glaze of colour with intense pearl. It can be used on its own or mixed into other products like foundations or moisturizers to enhance their iridescent qualities. Designed to apply easily and uniformly, it blends beautifully and can be applied on bare skin or over foundation. Because it's so concentrated, it should be used sparingly at first.