the ones that are made from gold and the others that are made from silver. Based on these metals as the basic material, a large variety has been designed from time to time. The gold varieties are considered to have good exchange value as they can be broken down into smaller pieces for the purpose of selling a portion without affecting the overall value in any way.
People have been buying bullion for centuries. In the past, all coinage was in fact bullion, made with pure silver or gold. Now the coinage used by most countries is simply coated in an alloy making the actual value of the currency worthless.