Quality index of silver is hallmark. The higher the hallmark the higher and better the silver quality and the more costly the coin made of this silver. The higher hallmarks are 945, 925, 903, 900, 810, 720, and 640. The coins with hallmarks under 500 are billons. They made of alloy consisting of silver and a base metal.

The reasons for using an alloy in coinage are: first - the hardness of alloy (remind that the silver is soft metal), second - the alloy make the coinage cheaper. It is possible …

Alloy
An alloy is a combination of two or more pure metals mixed together. All coins are made out of some measure of alloy, except bullion coins.
Bullion
Bullion is the pure form of a precious metal such as silver, gold, copper, or platinum, from which coin metal alloys are made. The U.S. Gold Eagle is not really pure bullion; it is an alloy of 91.67% gold, 3% silver, and 5.33% copper, although the total amount of gold content weighs out to a solid ounce. The U.S. Silver Eagle and the U.S. Gold …

The word originates from the middle Latin billo, meaning “a coin containing mostly copper”, or just “unit of payment”.
Billon is an alloy of a precious metal (most commonly silver) with a majority base metal content (copper). It is used for making coins, medals, and token coins.
Use of billon coins date through Middle Ages to ancient Greece. During 6-5th century BC, some cities on Lesbos used coins made of 40 percent silver and 60 percent copper. In Ancient times and the Middle Ages, leaner mixtures were adopted, with less than 2 …