Luster or lustre, sometimes called 'Mint Luster' or 'Coin Luster' is the sheen or reflective effect on the surface of a coin that is produced during the minting process.
Luster is useful in determining the grade of coins since it can’t be forged. It’s created when a planchet (a coin blank before it has been stamped) is struck repeatedly with a die, causing metal to flow from the centre of the coin towards the edge.
This produces microscopic parallel grooves or striations that reflect light in a unique, criss-cross pattern. When a coin is tilted under a light source, the pattern appears to spin around the surface of the coin in what is known as the 'cartwheel effect'.
Wear and friction can cause the cartwheel effect to break down from the centre out so it's advisable to wear gloves when handling your gold and silver coins.