The pebble count is a very important measure of substrate composition.
Sand is anything that feels gritty.
Fine Gravel is any particle that is 0.15 to 0.25 inches across. Small Gravel is any particle that is between 0.25 and 0.5 inches across. Large Gravel is any particle that is between 0.5 and 1.25 inches across. Small Cobble is any particle that is between 1.25 and 5 inches across. Large Cobble is any particle that is between 5 and 10 inches across.