A gem that is twice the size of another, otherwise equal, gem will be more than twice as expensive. That is because many more small pieces of rough gems are found than large one when mining. A gem is more valuable when it is more intense . If it is too dark, it becomes inky and dead, thus less valuable. If it is too light, then it isn't as attractive either.
Sapphires are a classic example.
You will see many sapphires that are so dark that they look like onyx from a couple of feet away. They aren't too expensive. You'll see a lot of them in jewelry being sold at a discount. A fine sapphire has the kind of blue that looks very blue , even at a distance. It'll never be mistaken for onyx.