I use a crommelin high gloss concrete paving sealer for my stones due to the durability but i skip the initial thinning step for first coat as i don't want it to soak into the stone too deep and 3-4 coats
But there are a lot of options, i have heard of nail polish, wood varnish etc.
I would have no hesitations in buying a stone with a line like that. i have a few myself
@mainaman - what do you use to lacquer your stones?
This stone has a line much the same through it but it came pre clued to the wood. It's hard to see but it is visible in the photo
