Firestone is an irregular chunk of phosphine-bearing rock. It is a mixture of gray coke, crystalline silicate and a reddish substance, which contains a high phosphine content. The phosphine mixes with second stomach acids and produces the gas, which allows the dragon to flame. Both the coke and silicate turn into ash, which is regurgitated later into the ash pit. When selecting stone to be sacked for flight, the pieces should be medium-sized, small enough to chew but not so small as to be unproductive. Discard stone with large amounts of silicate, as it is very hard to chew.

When filling a sack with firestone, bear in mind that the sacks will be tossed from one dragon to another. Weyrlings are expected to help pass sacks down supply lines, handing sacks from one weyrling to another until the sack is handed up to an older rider on a supply dragon. The older rider will fly into the air and toss sacks to fighting dragons during thread drills. When the Weyrlings get older and begin flying, they will be incorporated into the group of riders that toss sacks of stone to fighting dragons during fall or drill.

Weyrlings are expected to practice sack tossing and catching eventually standing a dragon length apart. This is an important skill to learn and you are expected to become proficient at it. Later you will practice sack tossing sitting on your dragons doing ground drills, then in the air.