In some mineral deposits gold occurs as fairly large inclusions in a harder matrix mineral. Gravity methods for separating this gold are generally those used in the aluvial gold industry (free gold that is recovered from gravel deposits). The rock that is mined must be ground sufficiently to free the gold from the matrix and then use is made of the difference in the specific gravity of the gold and the matrix to separate the two substances. Coarse gold, in this context, is gold which is larger than a few hundred microns making it visible to the naked eye.
Some gold gravity separation methods can be used to treat fine particles if there are large density differences between the desired and undesired minerals. In gold plants, for example, a number of gravity devices, old and new, are being used to recover relatively coarse gold. Over the past few years gravity separators that take advantage of differential specific gravities in a high-gradient centrifugal force field (e.g., Knelson and Falcon separators) have been used successfully for gold. Older devices (such as spirals on which the centrifugal forces are lower, pinched sluices, and Reichert cones) have been adapted for other heavy minerals.