My fireplace is about the same color rock as your's, and I have the same type of rock mantel! It was hard finding just the right things to put on it because it's in a dark corner. Light is key, do you have a narrow lamp you can set on it?

The color that pops for me is cream. For awhile I put my small collection of crocks on it and the color really stood out.

Now I have a largish picture sitting on the mantel leaning back against the wall, with only one thing on either side of it, one being a lamp, even though there's an overhead spotlight which I sometimes use.

When you set a lot of small things on the mantel, along with the dark clock, it all gets lost. It takes a lot of work decorating a mantel, something I thought would be easy. :)

I'll try to find a picture of my mantel tomorrow.