What is a rock that is rarer than a diamond, but not too ridiculously expensive?

My teacher was talking about a rock that he owns that is rarer than a diamond and was expensive, but not overly...what could it be?|||ruby|||Diamonds are not rare, it was De Beers that managed to corner the market on diamonds because Africa was the only continent where glaciers had not moved the surface deposits away from the underground vent where the diamonds are from. Quite literally, diamonds were just common rocks on the ground in Africa. There are three major vents bringing diamonds up from the core of the Earth, the African one was undisturbed by glaciers but the Canadian and Siberian ones where only recently located. Of course, now we know how to grow diamonds so not only will they be even more common but their purity and properties can now be controlled. Diamonds will replace silicon as the semiconductor of choice. Now the most rare rocks on Earth are the Lunar rocks brought back by the Apollo program.|||a flawless Emerald crystal