Pure silver is a little too hard to work with as its a little soft. So the material is usually mixed with copper or zinc to produce a mix with a slight impurity but a satisfactory level of silver to still be mostly made of silver.
The level of 925 parts per 1000 being silver is referred to as Sterling Silver, and is usually marked as such with a .925 Marking.
This is meant to differ from other levels of silver and is MEANT to provide a degree of security that the material is actually represented as sterling silver. These days you dont have to use chemical tests on the material .. you can actually use a specialist tool to check the composition of the material.
Sterling Silver as the Anchor Material
The material used in making your favorite silver necklace is not made from pure silver. Pure silver is soft and malleable, and therefore not ideal for jewelry. To address these limitations, jewelers usually combine pure silver with another metal alloy that is harder and can withstand use and abuse. Sterling silver is the most common alloy, and is made by combining silver with copper. Sterling silver consists of 92.5 percent silver, and the rest copper.
Copper Helps Bring Durability
Other metals can combine with silver, like zinc and nickel, but copper provides the best blend. It makes the alloy more durable without affecting the color of silver. The addition of copper into silver will not influence the overall value of the precious metal. What affects the overall value of silver is the labor and creativity that goes into making it, and the complexity of the design.