Yes, it is natural. 95% of what is called asbestos is mined as Chrysotile, the fibrous form of the mineral Serpentine. It is a hydrous magnesium silicate. A few other natural fibrous minerals are also called asbestos -- anthophyllite, amosite (grunerite), crocidolite (riebeckite), tremolite, and actinolite. Prolonged inhalation of asbestos fibers can result in mesothelioma, a fatal lung cancer, and asbestosis, a lung disease.

Encyclopedia John's answer is mostly correct. However, there is a dirty little secret that the alarmists don't want you to know, chrysotile (pronounce kris-oh-teel) asbestos is harmless. It's the other, less common types that cause the problems. There are three minerals that are used for asbestos, chrysotile, crocidolite, and amosite. The fibers of chrysotile are made when the flexible sheets of this mineral curl up to form tubes. These tubs remain very flexible even down to dust-sized particles. The fibers of the other two are long splinters of those minerals. When the are broken into dust-sized pieces, they become very stiff, inflexible slivers. When these slivers are inhaled, they can become lodged in the tissues of the lungs. (Think of a wood sliver that gets stuck in your finger.) Because these slivers become lodged in the actual tissues, the natural processes used by the lungs to expel contaminates can't get them out. Additionally, these mineral are not biodegradable. So, as you continue to breath more, more slivers build up in the lungs where they irritate these tissues until they cause serious lung disease. On the other hand, the flexible tubes of chrysotile don't become lodged in the lung tissues and they are biodegradable. So, these particle don't build up in the lungs and don't cause the irritation that causes the diseases. Studies have supported this analysis. These studies have shown a definite increased risk of lung disease in people that are exposed to crocidolite and amosite asbestoses. Sometimes it only takes a year for the damage to manifest itself. These studies were performed in people that lived around mines in South Africa and Australia. However, studies of people who lived near chrysotile mines showed little to no increase in lung disease. In fact, one study of women who lived next to a mine that didn't have any dust control on its operations found that they actually had a lower than normal occurrence of lung disease despite their very high exposure to chrysotile dust. So, a whole industry has grown up around a mostly none existent problem and has destroyed another industry. Because of a problem that comes from the least common forms of asbestos, all of it has been banned and a whole industry has grown up to remove even the stuff that is harmless. This is a prime example of a place where hype has over ridden reason. Ban the forms that are dangerous and promote the form that is safe. Don't throw the baby out with the bathwater. This information comes from the geology textbook from which I used to teach a college level geology class, Essentials of Geology, 7th edition, by Frederick K. Lutgens and Edward J. Tarbuck.

Nonsense, chrysotile is far less able to cause cancer, but it is not harmless. It is still hazardous from the asbestosis aspect (I have read of it having a half-life of 18 months in the lungs, which is far better than the other type of asbestos, but not exactly insignificant). Secondly, any given sample of so-called chrysotile is not necessarily 100% chrysotile, and may or may not have a medically significant contamination of a more cancer-hazardous form. The textbook mentioned earlier sounds as though the author was using Canadian asbestos industy standard PR information about their product and didn't check epidemiological sources. Please read around more thoroughly on the topic of asbestos. If you decide to read the Wikipedia article, also read the discussion page where users hash out and discuss some of the uncertainties or biases, so as to be less easily mislead by statements in the article which may or may not be fair in any given moment (as you know the Wikipedia articles are being edited all the time and that has known risks information-wise).