Lots of people come to Runswick looking for jet; fossilized monkey puzzle trees that have become a hard, black, easily carved and polished semi-precious stone. Here it is in two natural guises, both found by the ten year old Number One Field Assistant:
There are lots of pieces of promising black jetlike rocks on the beach, here's how to test if it's jet or not; jet feels warmer and less clammy to the touch than shales. Next pick up a piece of sandstone and scratch your suspected piece of jet along it.
Black mark; it's coal, chuck it.
White mark; some imposter rock or other, not jet, chuck it.
Dark brown mark; low-grade jet. Keep for interest or to give away as a present.
Light brown mark: high quality jet. Keep it.
More in depth stuff about Whitby (and district) jet at the splendid Yorkshire Coast Fossils.