Hi,

"Dep" is probably correct with thinset (modified).

If that flagstone is the least irregular on the bottom you may need a little thicker than normal "adhesive". Home depot sells that ,too, and refers to it as a "marble" product. It'll be right beside the modified thin set and costs more money. You'll need a 1/2" x 1/2" notched trowel to put it down instead of the normal 3/8" or 1/4" for thin set.

I would go to Johnbridge.com and post your question on that forum...It is the best website on the net for any tile question. They'll even walk you through the whole installation if you'll provide them what they need to know.

As "dep" said, matching the mortar will be what my Mom used to call a "boogerbear"!!!

Dave S.