Pyritic concretions (know to locals as “cannon balls”) can also be found throughout the shales in the Jet rock. However Pyritic concretions occur in many shapes and sizes in the different Beds. For example in Bed 19 the nodules are larger then normal with the largest I have observed being two and a half feet long by one and a half feet wide! It is in Bed 22 where we tend to find the almost perfectly round nodules and as such this Bed is known locally as the “cannon ball Bed”.
These pyritic nodules are generally made up of a pyrite skin (around half an inch thick) surrounding a limestone core. It is inside the limestone core that Ammonites are commonly found, usually these specimens are in a great state of preservation most being almost uncrushed by the vast pressures generated in the fossilisation process. This can be clearly seen in the picture attached the ammonite shown is the gorgeous Eleganticeras and a common find within these nodules.
Not all “cannon balls” will have Ammonites in them so it is advisable to crack them open (wear the appropriate safety clothing….) before lugging them back with you, they can get quite heavy after a while……!
The “cannon balls” are washed out of the Jet rock by the tide action and can readily be found all along the North Yorkshire coastline stretching from Robin Hoods Bay to Runswick. If you are lucky enough to find one, protect it from Pyrite disease by keeping it contained with a small bag of silica gel.
It is possible to prepare the pyritic crust to form a smooth polished finish on some ‘cannonballs’. The result is stunning, however the methods are dangerous due to the nature of Iron Pyrite. Iron Pyrite has been used as an effective fire-starting stone for centuries. When struck, Pyritic nodules give off sparks, these sparks are small pieces of Iron Pyrite that have broken off and instantly react with the oxygen in the air. It is therefore easy to see how grinding, cutting, polishing or breaking pyritic nodules can be a hazard. Take care out there people!
To identify a pyritic nodule is an easy task. Firstly, it will have a strange texture compared with other nodules, even if sea-washed these nodules are generally rough to touch. The second property you will notice is the density, they are much heavier than most other objects of the same size found on the beach, it will feel like you are holding a solid lump of metal. Upon striking the nodule with a rock hammer, you may notice sparks being emitted and also a strong smell of sulphur burning (rotten egg smell) The last clue will be within the nodule once opened. You will see a ring of gold pyrite around the outside edge of the nodule.