You may turn your stones into crushed stone (mixed gravel) at this gravel mill. Sand, fine gravel, and coarse gravel may then be separated from the crushed stone/mixed gravel. Coarse gravel may be further processed to produce fine gravel, which can then be pulverized into sand.
Input for Gravel Factory: Price: $25000; Stone Output: Crushed Stone, Gravel, Fine Gravel, and Sand
Sellpoint for Gravel and Sand Input: Fine Gravel, Sand, Crushed Stone, and Gravel Cost: $100

Credits: w33zl