Are blast furnace slag, sometimes called sea slag glass. To the north of Runswick lies Teesside where for 150 years there was an extensive steel industry; in less enlightened times the slag (the impurities that cook out of the molten iron) was simply dumped into the sea. (There was also a derelict ironworks on Lingrow cliffs to the north of Runswick. It opened in 1857 but collapsed into the sea in 1858. Pics to come, some if it is left standing.)
These blue-ish pebbles will have been washed along the coast and rumbled into a state of smoothness among sand and shingle. They are deceptive; they look lovely when you see them on the sand, wet and glistening from an ebbing tide. Pick them up, pocket them and get them home and they dry to a rather drab matt blue or grey. Here's an illustration under the rather drab lights indoors this evening. I grabbed eight pieces of sea slag from my miscellaneous specimen boxes very much at random, soaked four and left four dry.
These don't show the colours or swirliness particularly well; weather permitting I'll go for a walk tomorrow and photograph some specimens wet on the sunlit sand.
Andy from the excellent Andy's Fossils website (which you should visit and bookmark if you are at all interested in the Jurassic fossils found hereabouts) says much the same, and emailed a pic of cobbles made from the same material used on the path to the Cook monument in Whitby: