Mineralogy is a hobby of mine, so when I saw that shot, it struck me and then, of course, I went back and stared at it a lot. I know that it could just have been a casual prop decision, but the idea that it could be more kept tickling at me. For example, in HLV there are no close-up shots of the ring Sherlock showed Janine and the idea that he is proposing comes across clearly, even without her reaction and John's remarks. So it wasn't necessary to have such a close-up of John's ring to tell us he was planning to propose to Mary that evening, a glimpse of a ring in a box would have told us.

Perhaps someone will have an interview where that question is addressed and it will be clarified that it was just the ring that happened to be handy!

Oh and Mycroft's ring...yes, much mystery there, especially in light of "caring not being an advantage".