I am not a cover designer, by far not. I A-B-tested with my family the two options and started out with the "diamond" cover. After a few weeks, also seeing the print version of the book, I decided to make the cover a little bit more friendly.

The content of the book remained unchanged, but by just changing the outer appearance, it felt as if I sorted my book into a different category:
Black/Diamond: focus on stark, slightly haunting, mysterious
Pink/Thief: sunny, romantic, friendly
I haven't analyzed yet, which version brought in more readers.

I am not very happy with either cover, but so far they will do.
When "Brilliant Actors" will come out in 2014 (Book 2 in the Calendar Moonstone series), I will decide what way to go. Either play on the two existing cover themes or have a professional designer create me stylistically matching covers for book 1 and book 2