Relative sizes of gravel, sand, silt, and clay particles.

Beginning with the largest mineral particles, coarse fragments are greater than 2 millimeters in diameter, an dare primarily composed of unweathered or slightly weathered rock fragments such as gravel stones, and cobbles. They are relatively inert in the soil and are not considered directly when determining the USDA soil texture.
Next in size are sand and silt particles. Like coarse fragments, they may be unweathered or slightly weathered rock fragments containing primary minerals. They may also be made up of resistant primary minerals such as quartz that remain from the breakdown of larger rocks. Together sand and silt form the skeleton of the soil.

The green dot is representing silt with 0.05 to 0.002 mm. The clay is represented in red with less than 0.002 mm and sand is represented in tan with 2.0 to 0.05 mm.