If you're playing in the Isle of Siptah, the best way of getting Goldstone is by mining it from gold nodes with a pick or a pickaxe since you'll find them through several locations on this map (look for mostly nearby Vaults). The better the tool, the more resources you'll get from mining a rock. If you're playing in the Exiled Lands map instead, you must mine Obsidian nodes by the volcano in the north (Road of the Pilgrim is a convenient place (G-12 on the map grid)). However, Goldstone drops by chance from Obsidian nodes (it is a rare drop), so insist on mining these until you get the amount of gold you need while not forgetting that spending points in your character's Expertise attribute will help you collect resources more efficiently (look for unlocking the Careful Harvest perk). While still in the Exiled Lands, opting for killing Gold-vein Rocknoses by the volcanic area and harvesting them with your mining tools is quite profitable.