Fill a crack in the granite countertop
The typical thing to use when joining the two slabs would be epoxy, clear or tinted to match the color of the stone.

It's been a while since I've used it, so I don't know how easy it is to find 'pourable' epoxy that's intended for repairs after installation.  (we used to use it for making composite structures, as you can spread it really thinly).  You'd likely want to find the thinnest (least viscous) formula if it's a very narrow crack.