TCW means 'total carat weight' and is the total weight of all of the diamonds added together. Larger diamonds are more expensive than smaller ones and a single 1 carat diamond is significantly more expensive than two 0.5 carat diamonds. Some retailers try and make their jewellery seem more impressive, and more expensive, by increase the the total carat weight using small diamonds and charging a similar price to a necklace with a single, significant diamond. However, these retailers are actually offering poor value.