Colorado climber John Gill introduced the use of chalk (a.k.a. magnesium carbonate) to the climbing world in the 1970s. A former gymnast, he decided to try using chalk to absorb sweat while climbing. It worked well and caught on quickly. Unfortunately, because rock climbing has become such a popular sport, the use of chalk can have detrimental effects on the environment. Chalk forms thick deposits on the rock in wet climates, and the white splotches it leaves can be unsightly. Clouds of chalk in climbing gyms can adversely affect people who suffer from asthma. On the other hand, many companies are now making chalk that has coloring to match the rock, and often gyms disallow the use of chalk, but do allow chalk balls, which have less of an impact. Various types of chalk are available. These are the three most common forms.
Loose chalk is a magnesium carbonate that comes in plastic bags and can be poured into chalk bags. These are typically resealable bags.
Chalk balls are small, knit balls (either sewn or refillable with drawstrings) that control the amount of chalk you pull out of your chalk bag. Sometimes, if your chalk bag is tight, you will pull out more than you planned. Also, chalk balls ensure you won't lose all your chalk if you sit on your chalk bag or it turns upside down in the wind.
Chalk blocks are the least expensive form of chalk you can buy (because there is little packaging associated with the product). They are typically wrapped in paper packaging, and chunks can be broken off and placed into your chalk bag.