A blue diamond gets its natural color from an impurity known as ‘boron’. It is one of the rarest colored diamonds out there, which gets mined only in mining facilities of select countries. A fancy blue diamond ring will have a higher price, often ranging up to tens of thousands of dollars, as opposed to one that is color treated to imitate the former. The same goes for all of the natural colored diamond rings compared to artificially colored counterparts.
You might be thinking how a layperson would differentiate a fancy blue to a color treated diamond ring. The distinction will not be pronounced for an untrained eye, but there are specialized implements to tell apart the difference. Always rely on a lab report or GIA certification to avoid a fake purchase.