From the report:

"Execution by stoning, a punishment prescribed in Iran’s Penal Code, is a particularly grotesque and horrific practice. Amnesty International opposes the death penalty in all circumstances and believes that stoning is specifically designed to increase the suffering of victims. Iranian law prescribes that the stones are deliberately chosen to be large enough to cause pain, but not so large as to kill the victim immediately. It is a punishment meted out specifically for adultery by married men and women, an act that is not even a crime in most countries of the world, and the majority of those sentenced to death by stoning are women."