Once you've determined what cut, color, and clarity grade you're looking for in a diamond, it's easy to determine the carat weight of diamond that will fit within your budget.
When diamonds are mined, large gems are discovered much less frequently than smaller ones, which makes large diamonds much more valuable. In fact, diamond prices rise exponentially with carat weight. So, a 2-carat diamond of a given quality is always worth more than two 1-carat diamonds of the same quality.
To choose the best carat weight of diamond, consider style, finger size, setting size, and budget.
If you have a set budget, explore all your options and you'll find that there is a wide range of diamond carat weights and qualities available to you.
If your recipient is very active or not used to wearing jewelry, she may find herself bumping or nicking her new ring. Consider a smaller size diamond or a setting that protects a larger diamond from getting knocked against doors and counters.
Also keep in mind that the smaller the finger, the larger a diamond will appear. A 1½-carat diamond solitaire looks much larger on a size 4 finger than a size 8 finger.
Finally, if a large carat weight is important to you, yet you're working within a budget, consider a diamond with a good cut, SI1–SI2 clarity, and an I or J color grade.