What is the most expensive gem to buy?

I'm curious to know, when it comes to precious gems, which one holds the title of the most expensive to purchase? Is it a rare diamond, perhaps boasting exceptional clarity and carat weight? Or could it be a stunning emerald, known for its vibrant green hue and historic allure? Alternatively, is it a sapphire, renowned for its royal blue coloration and legendary status as a gemstone of royalty? The question begs to be answered - what is, indeed, the most costly gemstone to acquire in today's market?