What is the rarest gem on Earth?

Can you shed some light on the question of what is considered the rarest gemstone on our planet? Are there any specific criteria that make a gemstone exceptionally rare, such as its rarity in occurrence, the difficulty in mining, or its unique aesthetic qualities? And furthermore, if we narrow down the field, are there any contenders for the title of the rarest gemstone on Earth that stand out among the rest? It would be fascinating to hear your insights on this topic.