What type of compound should I use for granite countertops?

I had granite counter-tops installed in my kitchen 2 years ago. It came in two pieces, they are joined together at the sink. For some reason the two pieces have separated by just a small fraction, but enough for the compound that separates them to come apart. The compound seemed to be colored to match the color of the counter-tops. What type of compound should I use to fill it in? Obviously it needs to be something that won't degrade in water, since it is right near my sink.