Placer deposits are grains and fragments of gems and metals that have been transported, usually by streaming water, from the source of origin and deposited by gravity separation through sedimentation. These can be found in sand and streaming water near the original source. It can be mined by placer mining where you use a prospector’s pan.
You have to be in a river or beach biome to try placer mining. The method is done by breaking block with the prospector's pan. There is then a chance that gold nuggets drops, or even diamonds.