Have used linseed oil before on floor bricks before on a old gaff, just thinned it out with a bit of white spirit first.
looked good, was just a bit tacky for a day or two but shouldn't be a problem with a fireplace though.
A proper stone sealer will obviously also do the trick, just bloody...
Thought I better introduce myself as have just posted on plasterer looking for plasterer and bound to get slated if I don't!!!
Been plastering 17 yrs and took plunge couple yrs ago and went it alone.. and aint looked back!!
Bury St Edmunds based, aint married but got 3 kids with her so...