word meaning "appearing synthetic"?

Solution 1:

I'd suggest unnatural. Merriam-Webster defines it:

not being in accordance with nature or consistent with a normal course of events

I recommend artificial

Made or produced by human beings rather than occurring naturally, especially as a copy of something natural.

The 'natural copy' part seems a perfect fit re your rock formation example.

From Oxford Dictionaries

Alternatively, there's bogus

Not genuine or true (used in a disapproving manner when deception has been attempted)

Same source.