A local banjoist has one of these and it has an ebony nut. That really means nothing, of course. Nuts are easily replaced...esp. over a century. Bone was used, ebony, dyed hardwood (like maple or apple/pear), etc., etc. I would spend more time worrying about what "looks good" rather than total correctness (which is probably unknowable).
Generally, the nuts on this era of banjos are very narrow compared to today's stuff.
For me and a 'pearl' like this, I think I'd opt for a dash of contrast. Or not, Stewmac offers a pearl blank too.