Why do pure metals need to be mixed with other metals?
You will find examples of jewellery made of pure metals like platinum and palladium, but pure gold is soft and isn't practical for daily wear. Other metals are mixed with gold to make it more durable (and to lower its cost). For example, palladium or nickel can be added to pure gold to create white gold; adding copper produces a rose or pink tint; while silver gives gold a greenish tint. Even though gold will be in different carats it is still classed as solid gold.
Gold-plated jewellery is when a base metal (copper, brass, nickel, silver) is covered with a thin layer of gold, giving a great look but one that will have to be maintained. Gold-plated jewellery can be the biggest source of allergic reactions due to the plating wearing off and the base metal being exposed to cause irritation. Satyam Jewellers uses Sterling Sliver as the base metal to reduce the risk of allergic reactions.