Sterling silver is made of a mix of metals with the silver content being 92.5%. That's why you often see 92.5 stamped on silver jewellery pieces.
When you mix metals like this to make a new one its called an Alloy. The other metals in the mix are most commonly copper but can also include zinc, nickel or platinum. The reason that these other metals are included is to increase the strength and hardness to make it more durable, as silver is too soft to use to make jewellery on its own.
We use 100% recycled silver in our sterling silver.