Black Rubber

In industrial marble floors the grout lines look like black strips of tar/rubber? Is it? What kind of grout?

I want to marble my foyer. In hospitals the large marble tiles look like the grout lines are a thin strip of something black, rubber? It is not a sand type grout, what is it????I want to avoid sand grout because of how dirty it gets. Suggestions?

On one of the jobs I was on the tile people used a two part grout and I would not want to try that myself but it looked great and turned out to be bullet proof. Cleaned up well and was water proof.

Synthetic Adrenaline Music- Black Rubber Dance / Preaching to the Perverted