Simply put, larger stones are less common than small ones. Hence, they demand a higher price per carat. For example, a quarter-carat topaz may cost $60 per carat, or $15. A half-carat topaz, (with the same color, clarity and cutting grades,) might cost $100 per carat, or $50. A full carat topaz would cost $200.
Choosing the right size is a personal matter. For the bold, dynamic individual, a large gem mirrors their personality. On the other hand, small stones are better suited to someone with delicate and feminine tastes. Most people will fall in between these two extremes.
Then budget is a strong factor, smaller stones have a significant advantage. Not only do they cost less per weight, the amount of gem you see is disproportionate to their size. The reason is that volume goes up faster than the outside dimensions. For example, a half carat, round diamond measures 5 mm in diameter, a ¾ carat diamond 6 mm, and a full carat 6.5 mm. From a casual observation, the half and ¾ carat stones, or the ¾ and full carat stones look to be about the same size, but the price difference can be considerable.
Small gems are often clustered to give the illusion of more gemstone. Seven 1.6 mm diamonds, set close together, will take up as much space as a whole carat diamond. If set on white gold, it is hard to distinguish the separate stones, hence these are often called "illusion settings". For astrological purpose a single diamond is necessary and not cluster of small diamonds making up the weight.
While these seven stones approach the eye appeal of a one-carat diamond. They only weigh .14 carats. Considering that the price per carat is also much lower, the cost difference is significant. A cluster ring would be in the hundreds of dollars, rather than the thousands.
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