Are you putting this in your home?

The first one, which is not ornate, might look nice with a wood mantel. Warm paint would also help. Maybe even an accent color a little darker than the rest of the room above the fireplace. Can't think of anything to help with the marble one.

I agree about adding an accent paint color to draw your attention to the fireplace. Adding a decorative mantle and nice piece of art or mirror will spice up that space and turn it into a focal point rather than an eyesore. I like the mantle of the second one, it has character. Adding a warm neutral color and a decorative screen across front could be a nice addition as well.

definitely paint the wall above the first fireplace an accent color. u want to warm up the space, but the colors of the dark stone don't seem warm--are they? if they are, u could pull out an accent color from there. if not, there's a lot of greenery visible out the windows next to the fireplace--u could either bring the outdoors in by choosing a more modest shade of green, adding an organic sense to the space, or pull out the color of flowers outside for your accent color. as for the 2nd fireplace, the mantel is refined and classical but the collection displayed above seems more rustic asian--2 competing styles that seem difficult to pair. i think u need to go w/ a clean, simple accent, like a mirror or painting in a frame that won't compete w/ the style and lines of the mantelpiece, or u need to change the color and style of the mantelpiece.