Shims are available in Aluminum and Thermo Plastic, despite the latter being a lighter solution its material characteristics limit the thickness of the ID - OD measurements. Aluminum is used to achieve the thinnest shims as it is a more robust material.
Rolled Aluminum shim - the very thinnest shims are rolled from sheet metal. 25.4mm shims need to be so thin as they are etched down in acid to the correct tolerance and are too thin to anodise (show as silver).
Machined Aluminum shim - machined to very high tolerances from 6082 T6 Aluminum and then black anodised to protect from corrosion.
Machined Plastic shim - larger size shims are machined to very high tolerances from "filled" engineering thermoplastic UPVC. This keeps the weight down, as the shims get thicker.
Shim design characteristics may affect weight.
Weights are accurate to the nearest gram.
Shims allow a rider to adjust the ID of a frame for a range of different sized posts.