How long and how wide is it?
What product can I use to seal a crack in our (marble) shower wall. Previous owners used caulking and it's not durable. In fact it's peeled off.
What is the widest separation? This may be a candidate for some epoxy. Done right, it might be invisible, done wrong, it could ruin the wall. You might want to talk to a stone fabricator/installer.
Important note - I'm not a pro
Retired Defense Industry Engineer; Schluter 2.5-day Workshop Completed 2013
Epoxy colors match marble well. Epoxy works. I have done this.