The Stone of Farspeech is basically a fantasy walkie-talkie or, if you prefer, a fantasy smartphone.
Originally, Magnus bought three of them from Fantasy Costco and gave them to Merle and Taako to help communicate during missions. Eventually the Director and Angus use them to communicate with the Reclaimers during missions. Merle also gives Stones of Farspeech to his son, Mookie, and step-daughter, Mavis, to keep in better touch.
They were voluntarily given to and destroyed by Barry Bluejeans, so that they could communicate in private.