The name given to rounded, water-worn stones produced by rock attrition by moving water.River gravels generally form in the upper reaches and in the middle of the stream. Both kinds consist of hard types of stone, flint, granite and sandstone, the color of the gravel being governed by the rocks from which it is formed.
The sizes of the individual stones vary and from gravel beds the stones are graded and can be purchased as such. Frequently sold as ballast, the best size is a grading from 19-5mm (3/4 3/16 in). When gravel alone is used to surface paths it is loose and unstable, noisy and sticks to the shoes in wet weather. For this purpose it should be mixed with enough sand to allow it to bind together to form a firm, smooth surface (see also Paths in the garden).
The main use to which the gardener puts gravel is to cover the surfaces of greenhouse staging, and to fill gravel trays in which to stand pot plants. The advantages of using such an aggregate are the provision of nonabsorbent material, dust free, weed free and of good appearance. It can be sprayed over in the process of damping down and the water evaporates from such a surface.
The gardener uses slightly coarser gravel as an aggregate in the ring culture of tomatoes, for growing such plants as water cress in suitable troughs and for stabilizing plants during soilless culture or hydroponics. Similarly bulbs can be forced indoors supported only by gravel as a medium and the water level maintained so that the stones are just covered. Narcissus ‘Paper White’ succeeds happily in this way indoors.