A gold ring that is 18 karats (18k) indicates that it is made of 75% pure gold, with the remaining 25% consisting of alloy metals such as copper, silver, or zinc. The purity of gold is denoted by the karatage, with 24k being the highest and purest form of gold. An 18k gold ring is popular for its balance between purity and durability, making it suitable for everyday wear while still retaining a significant gold content.