A triangular mantel literally fits the shape of the wall and forms a flat, triangular surface. Instead of placing artwork on the walls that form the corner, consider placing a large plant, a bronze statue, or a grouping of candles onto the mantel. You might also visually create height in the center of the mantel surface by placing a tall object in the center, and shorter objects on either side to draw the eye up, then down. Paint the walls behind the mantel a light color to visually pull them forward and open up the space.
A diagonal wall is one in which the fireplace is not only placed on the diagonal, but the mantel is as well, with no triangular surface space. The mantel has a straight, rectangular configuration. The wall above may be diagonal as well to match the shape of the mantel. This is ideal, as this setup allows for optimal placement of artwork just above the mantel. A large framed painting or elaborate mirror works well in this space and creates a focal point in a room. A flat-panel television may also be installed above the fireplace on the diagonal wall. The flat wall above the mantel may be painted in a contrasting color to further the focal point effect and make the fireplace stand out in the room. Use a balanced arrangement on the mantel itself; place one candlestick on either end of the mantel and a silk or real plant or elegant clock in the center.
Sometimes even with good design ideas, a fireplace needs a remodeling. Consider refacing the surround on the fireplace with new tile, brick or slate. Add a new mantel if the old one has seen better days. Refinish an old mantel if it is still in good condition; sand it down lightly and apply a new coat of stain and polyurethane. Existing tile or brick may be painted in neutral tones to update an old fireplace as well.