Found the original fireplace mantel half buried behind the garage.

Another before:

It's green Vermont slate, figured out that it was originally painted black and marbled. Victorian thing, faux marble mantels. Fireplace is also Victorian faux, red brick, would have had logs and a red light. I'll be putting in a gas insert at some point.

Started at 220grit, and worked up to 1000. Finished with a 50/50 mix of boiled linseed oil and mineral spirits:

Didn't realize slate could be this pretty and figured: