As a follow-up on suggestions, we acquired a pizza stone.
Here's the item. The stain you see are leftover bits of cheese that instantly cooked and turned to charcoal. It doesn't affect the taste.

Here's the result:

The pizza cooked more rapidly. My greatest surprise is that the bottom of the pizza shows no darkened marks. It's cooked - just
not burned. Also, the crust is chewier and tastier. I'd recommend a pizza stone.
Anybody else has experience with a pizza stone?