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This specimen of ironstone or magnetite (under plane polarised light) is from the now disused West Mine Grid Ref. SE729 945, (aka Low Works or Hollins Mine) on the west side of Rosedale, North Yorkshire. The hand specimen was able to deflect a compass needle, hence its name. Here, the magnetite (Fe3O4) has wholly or partially replaced the original berthierine or chamosite ooliths. According to J.T.Greensmith in his "Petrology of the Sedimentary Rocks", the replacement of the berthierine may have occured in the soft sediment under weakly oxidising to mildly reducing, alkaline conditions, and the change may have been promoted by certain varieties of bacteria. The cores of some of the ooliths still have goethite (?) preserved. It will be noticed that some of the ooliths have been broken up. A hint of the original concentric structure can still be seen in some of them. It was near Hollins farm that the working of ironstone in Rosedale began in 1856. Samples of ore from the initial quarry were found to be of very high quality. The ironstone vein was about 18ft. thick and was easily worked by drifting.
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