What kind of black snake has fangs and light gray diamonds on its belly?

It is difficult to tell without a picture of the snake, or without knowing which part of America you saw the snake, but if you live in the southern states, you may have seen a Mississippi Green Water Snake (Nerodia cyclopion).

Although named for its dark green color, the Mississippi Green River Snake often appears black in strong sunlight, especially if the snake is wet from a lake, pond or slow-running stream, which are among this snake's preferred environments.

As well as Mississippi, the Green River Snake can be found throughout Missouri, as well as parts of Alabama, Arkansas, Georgia, Texas, South Carolina, Kentucky, Florida and Indiana, with some sightings in Illinois. Areas containing marshes, cypress lakes and river valleys are good sites for the Green River Snake, which enjoys living in places of still or brackish water.

The Green River Snake is not venomous, but can give you a very nasty bite if it feels threatened, so it is best observed from a distance. They also give off an unpleasant musk if handled.

River snakes are fascinating creatures and if you manage to record some footage of one, as this YouTube has done, someone will be sure to make a positive identification: