The book is 70s or 80s contemporary setting (I read in 80s). The characters are somehow 'changed' or 'copied' by gems that are buried under ground. The main character was changed / duplicated as a child by two such gems. He has the ability to (gradually) change his appearance, as well as some other abilities I can't recall. It was more thriller than sci-fi.

The book takes place in a 70s or 80s contemporary setting (I read it in the 80s). The characters are somehow 'changed' or 'copied' by gems that are buried underground. The main character was changed / duplicated as a child by two such gems. He has the ability to (gradually) change his appearance, as well as some other abilities I can't recall. It was more thriller than sci-fi.