Stone, alongside Iron, is one of the two minable resources in the game. Stone is a finite resource found on some tiles. To obtain stone you first need to colonise the tile and then build a mine. You can increase your yield of stone through the Lore tree and via worker upgrades. The "Mining Efficiency" Lore is the only bonus that will increase the amount extracted. All others increase the rate of extraction.
Stone is used to upgrade existing buildings, usually in amounts of 10 or 5. The "Carpentry" lore can decrease the amount required.
Amount extracted per month:
Picture and data taken from: