Average Gravity of Some Metals
Mica: 2.3
Feldspar: 2.5
Quartz: 2.7
Calcite: 2.7
Hornblende: 3.2
Garnet: 3.5
Topaz: 3.5
Corundum: 4.0
Magnetite: 5.2
Silver: 7.5
Lead: 11.3
Gold: 19.2
Due to the fact that it is insoluble, the finest particles of gold are preserved and since gold is so malleable it will get hammed down into different shapes because of the stones that bang into it as it tumbles along the stream. However, it does not weld itself together and turn into bigger nuggets, as some people believe or wish were true. Pieces of gold might break down but this will happen from another piece. It has been shown that the biggest pieces of gold come from lodes and not from placers.
The rounder and flatter the nuggets are that you find probably mean they have been in the stream for a longer amount of time and have been knocked around more than the ones that come in the crystalline form. Crystalline nuggets are courser gold that has most likely not traveled as much from its original source.