"Horsetail contains silicon, potassium, aluminum, manganese, saponins, phytosterols, phenolic acids, cafeeic acids, alkaloids, and tannins. Fifteen types of bioflavonoids are also present. These bioflavonoids are believed to be responsible for horsetail's strong diuretic action. The high silicon content of the herb strengthens connective tissue, ligaments, bones, hair, and fingernails." http://findarticles.com/p/articles/mi_g2603/is_0004/ai_2603000434/
"Horsetail (Equisetum arvense), an herbal remedy dating back to at least ancient Roman and Greek medicine, was used traditionally to stop bleeding, heal ulcers and wounds, and treat tuberculosis and kidney problems. The name Equisetum is derived from the Latin roots equus, meaning "horse," and seta, meaning "bristle."
Today, horsetail continues to have medicinal value. The plant's stems are rich in silica and silicic acids, which help mend broken bones and form collagen, an important protein found in connective tissue, skin, bone, cartilage, and ligaments. Therefore, horsetail is used as a supplement to treat and prevent osteoporosis." http://www.umm.edu/altmed/articles/horsetail-000257.htm
I really don't like horsetail grass (shavegrass). One of the alkaloids in horsetail is nicotine, which causes constriction of the blood vessels. This makes it dangerous in high doses or for people with poor circulation to begin with. These include diabetics, those with Raynaud's disease or phenomena, people with congestive heart failure, and those taking non-steroidal anti-inflammatory drugs.