18k (75%) gold is the purest form of gold used for jewelry (pure 100% gold is 24 karats by definition). 18k means that 18 out of the 24 parts that make up the gold are pure gold. The most common hallmark stamps for 18 karat gold are 18K or 750 that indicate 75% gold purity. Rose gold is an alloy also known as pink gold and red gold. Although the names are often used interchangeably, the difference between red, rose, and pink gold is the copper content: the higher the copper content, the stronger the red coloration. 18K red gold means 75% gold and 25% copper. 18K rose gold means 75% gold, 22.25% copper and 2.75% silver. 18K pink gold means 75% gold, 20% copper, 5% silver.