Sterling Silver is 92.5% Pure Silver
The term sterling silver is used to describe 92.5% pure silver plus 7.5% of at least one other metal. It's not uncommon to see 925 silver being advertised as 'solid silver' which is completely incorrect.
Fine .999 silver has a purity of 99.9% with less than 1% other metals. This type of silver is extremely soft which makes it unsuitable for use in many household items and most jewellery.
To add strength and make silver more durable it's alloyed with at least one other metal. The silver must retain its appeal and can't lose quality or value so care must be taken when deciding what type of alloy to use. Although copper is the most common there are several others. The reason for creating different alloys is to try and improve resistance to tarnish and eliminate risk of firescale. Pure silver is highly resistant to tarnish but copper is not. Although new alloys are frequently introduced one has not yet emerged to replace copper as the industrly standard.
People with an allergy to sterling silver in the vast majority of cases are actually allergic to nickel which is often used as part of the alloy. Hypoallergenic silver is nickel free.
Using more than 7.5% copper not only reduces the value of the silver but also causes it to tarnish more quickly. The more copper that's present in the alloy the darker the silver.
Most countries around the world have their own system for hallmarking sterling silver. Hallmarks are used to indicate purity, to identify the silversmith or company who made the product or to document the date and/or location of manufacture. 925 which is the most widely recognized international hallmark confirms the purity of the silver is 92.5% with an alloy of 7.5% copper and/or another metal.