Often when in the field, it is useful to bring back samples of rock. To keep samples labeled and contained, each sample is put into its own sample bag, which is then labeled and securely tied shut.
I have used both cotton and synthetic sample bags (the one shown above is synthetic), and generally prefer the cotton. The bags are a bit sturdier, and with some of the rocks I sample being fairly pointed, they hold up more nicely in shipping. Sturdy cotton sample bags are also a bit heavier, so on expeditions where every ounce matters, the synthetic may be the bag of choice. My samples are also generally dry, but in wet environments the cotton sample bags may not be appropriate as they may degrade during shipping.