GE #1 Silicone sealant if you want to go with silicone. Be sure to get the clear. It also comes in white and the packages look very much alike. (Voice of experience here.)
You can also use Crazy/Super Glue in any of its forms. There's an aquascaper who does YouTube videos who likes to use cigarette filters as a kind of glue padding to help hold together rocks that don't touch over enough area for the glue to hold. Apparently you can buy them in bulk for cheap. (I just checked. 200 unbleached cotton filters for $6 delivered on ebay.) They soak up enough of the glue to solidify when set.