A completely smooth, uncoated, single slab of granite can indeed be koshered1. Here is what you would need to do:

a) Scrub it completely clean.

b) Do not place hot items on its surface for 24 hours prior to the koshering.

c) Pour boiling water over the entire surface of the (completely dry) countertop.

d) Run a hot rock or brick, or an electric iron, over the countertop's surface so that the water on the countertop reaches a boil.

For safety purposes, if you use an electric iron, make sure that the cord is unplugged before you use it to kosher the counter. Also, no matter what you will use, boiling water and hot rocks are dangerous. Plan the logistics in advance and exercise appropriate caution!

This procedure renders a counter kosher, and can also be used to kosher a chametz countertop for Passover. Nevertheless, for Passover purposes only, many will cover the counter's surface for the duration of the holiday.

Yours truly,

Rabbi Menachem Posner