Golden rocks obtained by training Construction are part of the Statue of Rhiannon. Two are required each to complete the statue each time that it is built. Golden rocks may be stored inside a statue collection bag.
Working on furniture or any construction activity that yields experience in the Construction skill, including in building mode and creating flatpacks on a workbench. Limestone fireplaces are recommended as limestone bricks can be acquired for 21 coins each from the stonemason in Keldagrim and sawmill operator in Prifddinas. They may be obtained while using Protean planks, which make a favorable option as they stack in your inventory. To conserve resources it is advised to build something that uses low amount of planks.
Use the Crystal Saw to double your chance of getting a rock.