Horsetail is a very old plant. Botanists say horsetail has been on this planet, basically unchanged, for about 100 million years. As you might guess by the name, it somewhat resembles a horse's tail (or maybe a pipe cleaner), and it's unusual in several ways. It reproduces through spores, like a fungus, instead of by seeds. Its leaves are tiny and don't produce chlorophyll; instead, its stem is responsible for photosynthesis. The stems are also abrasive because of a mineral coating composed mostly of silicates.
Those minerals make this plant's extract valuable for skin care, as the magnesium and silica help to build collagen and elastin. It is also an astringent, and valuable for hair care because it stimulates the scalp and prevents dandruff.