Cork granules are made from the bark of the cork oak tree and there is a wide variety available in both granule size and density. The cork granules from the insider layer of the bark are low-density, soft and elastic and in high demand, whilst the granules from the outside layer of bark are high density, more woody and brittle. There are also a wide rnage of intermediate densities available and each density is suited to different applications. Cork granules are also divided by size from 0.2mm to 12mm in various different bands and cork powder is also produced as a byproduct of cork manufacture.
The lowest density cork granules (55-65 kg/m3) are generally used for wine corks and bar-top corks, internediate densities (70-165 kg/m3) are used for example for decorative products, cork lids and cork flooring and high density granules (170-250 kg/m3) are use mainly for outdoor usages, such as infill for artificial turf and in agriculture. The price of granules reduces steeply as the density increases and you can read more about cork granule densities here.