"Nobody is exactly sure how nature formed them but in the absence of other evidence, it is assumed they are natural"
Kinda like the martian blueberries. No one who can be taken seriously thinks they were produced artificially but Mars is covered with them none the less.
We have a nice 3D picture of some of those from one of the Rovers. Of course, I call them Martian Moquis because I didn't hear the phrase "Martian blueberry" for a long time after I'd first seen them.