AKA magnesium (II) carbonate n-hydrate, magnesite, or chalk.
Chemical formula:
MgCO3 or MgCO3 • nH2O
Magnesium carbonate is a white powder found in dolomite as calcium magnesium carbonate ( CaMg(CO3)2 ) and has a wide variety of uses.
Around the house, magnesium carbonate can be found in icing sugar, salt, and other granulated foods as an anti-caking agent. It is also used in toothpaste as a mild abrasive. It's in the medicine cabinet as an antacid, in the kitchen as in ingredient in silver polish and in the bedroom in cosmetics. It's even in cigarette paper to help control the rate at which it burns.
In the gym, it's used as chalk to dry the hands for weightlifting, gymnastics, and rock climbing.
In taxidermy, it can be used with a 3% hydrogen peroxide solution to bleach bones for display.
Industrially, it is used in paints and ink and in paper for its whiteness. Magnesium carbonate is also used as a stabilizer, reinforcing agent, anti-bleaching agent, anti-caking agent, alkaline buffer, filter material, and many other things. It is also as a coating for high-temperature machines because it is very non-reactive and does not burn or melt.
With all these uses and more, magnesium carbonate is fortunately both very safe and extremely abundant. It can be acquired easily and cheaply and is relatively safe to handle as long as one is careful not to get it in the eyes, where it is an irritant. Flushing the eyes with water is recommended. Magnesium carbonate can be toxic, but must be ingested in very large amounts for any ill effects to be seen.
Sources:
http://education.jlab.org/itselemental/ele012.html
http://www.chm.bris.ac.uk/webprojects2001/anderson/anticakingagents.htm
http://www.sefsc.noaa.gov/HTMLdocs/MagnesiumCarbonate.htm
http://www.vandykestaxidermy.com/product/01347749/
http://www.a-m.de/englisch/lexikon/magnesit.htm
http://www.solvaychemicals.com/products/product_line/0,5408,-_EN-1000079,00.html
http://www.infochems.com/product/productinfo.asp?product_id=55650