Let's be honest. Most of us have no idea what we really want. Today's jewelry advertising tells us that women want at least a 1.00 carat diamond engagement ring. Hate to break it to you, but this is just a marketing gimmick. The difference between a round brilliant cut 0.95 carat diamond and a 1.00 carat is incredibly slight. 99% of people will not see a difference in size. However, you will see a large difference in price. Once a diamond reaches that 1.00 carat mark, the price jumps up. This carat weight is so sought after that the price is raised because of popular demand. But I'll let you in on a secret. If you stay just below that mark, you will be getting essentially the same look for a smaller price tag.
It's also important to keep in mind that a 1.00 carat round brilliant cut diamond is not that same as a 1.00 carat princess cut or pear shape diamond. Some fancy cuts are cut deeper or more shallow than a round brilliant cut, so they make look larger or smaller. Carat is a measure of weight, not of size.
Similarly, a 1.00 carat diamond is not the same size as a 1.00 carat sapphire. Each gemstone has it's own specific gravity. For example, an average 1.00 carat diamond measures approximately 6.5mm in diameter. A 6.5mm sapphire weighs approximately 1.25 carats. In other words, a 1.00 carat sapphire will look smaller than a 1.00 carat diamond.