Wonder

A natural wonder, commonly referred to as wonder, is an ancient term used to describe valuable thing that is made, or occurs naturally. To be more specific, only uncommon grade or higher natural object is known as natural wonder. The commons are simply known as natural objects.

Grade
Along with artifact, wonder is graded using the same system. But due to their nature, there are lesser highly graded wonder compared to artifact. According to multiple research, the higher the grade, the larger the disparity. For instance, for Legendary grade, the ratio of wonder to artifact is 1:10000 while for Mythical it was 1:5000.

Trivia

 * It is the antonym of artifact, which is used to refer thing that is made by humans.