Gold nuggets are formed when native gold has been formed into them from flakes or grains due to the force of water as with in a river or large body of moving water. In some instances, depending on the river the presence of gold nuggets will signify that a larger vein of gold may be present in the area. However, most rivers washed the nuggets downstream significantly and therefore finding a nugget will not mean the main vein is nearby. In other instances, if you are able to find a few pieces of gold near an area that has recently opened up or has had a landslide it may mean you are very close to the original vein.
Nuggets are worth more than worked or melted pieces of gold of equivalent size due to it being in a pure form. Some individuals who find nuggets are able to receive more in payment for finding the nugget than others would if they sold a piece of jewelry of the exact same size unless it was a historical piece. Gold nuggets are also able to be found more easily out in the open than gold flakes or gold dust due to being a higher concentration of precious metal.
In the era of the gold rush, nuggets would often be brought in to be evaluated in sacks which contained all of the newly discovered nuggets. These were then evaluated for their weight if not discovered in dust form (grains) which would then be melted down to evaluate their weight. Most nuggets were very small in size although a nugget was found in California which was over 150 ounces in weight. Although it is more difficult to weight gold nuggets, they remain nonetheless one of the most sought after and valuable forms of gold.