Diamonds are actually one of the most common gemstones and are not actually all that rare, but companies limit supplies making them seem rare and very expensive (just a random fact off the top of my head).
Could we have more leeway on the Cullinan question? Looking into whether the owner is the monarch/crown or the person currently holding that title, the answer seems vague. Basically, "owned by the monarch personally, BUT..."
If compressed enough, peanut butter will become a diamond or multiple diamonds. However, they would not be proper diamonds because of the composition from the peanut butter and would not be valuable. It is also inconvenient, because the pressure required to transform peanut butter into diamonds in much more than anything that could reasonably be performed for commercial use.
Please accept "round" for diamond cut, "round brilliant" is more technically correct as round can also refer to older styles of gem cutting, but most people use round to mean round brilliant... or have never heard of round brilliant before taking this quiz
as well as the country in which the largest diamond was found
https://aqua.org/explore/animals/diamondback-terrapin