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Whinstone was formed during the Carboniferous period and made mostly of Plagioclase Feldspar and Clinopyroxene along with smaller amounts of other minerals such as Biotite.

Credit: Simon Copsey <simoncopsey@cedstone.co.uk>

“Permo-Carboniferous Whin Sill Complex, Great Whin Sill Whinstone The Great Whin Sill is intruded into the Alston Formation and crops out in Northumberland as a discontinuous ridge, extending from the northeast corner of the county (Fenwick, Belford, Bamburgh), via coastal areas (Beadnell, Embleton, Craster), to near Longhoughton and south of Alnwick. The sill runs in a near-continuous belt through south-central Northumberland, stretching from Kirkwhelpington westwards (north of Hallington, Humshaugh and Newbrough) to Greenhead. The outcrop of the Great Whin Sill is typically represented by a linear topographic high, and this produces dramatic settings for the several prestigious structures built upon it, such as Bamburgh Castle. Figure 20: Bamburgh Castle, built on top of the Whin Sill. 19 The sill rocks (Whinstone) comprise dark grey-black to black, fine to medium-grained, weakly porphyritic, quartz-dolerites, with grains up to 2mm in diameter. The rock contains small phenocrysts of plagioclase feldspar, with pyroxene (mainly augite, hypersthene and pigeonite) set in a dark grey, fine-grained groundmass. Significant amounts of opaque iron oxides and pyrite are also present. Many rocks have interstitial quartz-alkali feldspar intergrowths. There is a marked decrease in grain size at the margins of the sill, where the rock comprises a very fine-grained tachylite. In the upper half of the sill, the dolerite is locally cut by flat-lying sheets, veins and segregations of coarse-grained pegmatitic dolerite (with crystals up to 20mm in length). Figure 21: The Blue Bell Inn, Embleton. Whinstone. Figure 22: The Victoria Hotel, Bamburgh. Whinstone with Stainmore Formation sandstone dressings and quoins. 20 Whinstone has been employed only occasionally as a local building stone (at Bamburgh and Embleton, for example) and in stone walling near accessible outcrops. However, its hardness and resistance to erosion make it an excellent roadstone and aggregate. Whinstone was quarried from several locations at Embleton, Bamburgh, Belford and Craster, and it was worked for concrete, roadstone, aggregate and building stone at Kirkwhelpington (East White Hill Quarry), Divet Hill Quarry and Keepershield Quarry. It was also extracted from large quarries at Barrasford and Colwell (Swinburne Quarry), although these were worked mainly for roadstone and aggregate.” From https://historicengland.org.uk/images-books/publications/building-stones-england-northumberland/bse-northumberland/

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References:

Further reading

https://www.cedstone.co.uk/

Industrial minerals, geology and man, Northern England – MediaWiki

https://earthwise.bgs.ac.uk/index.php/Industrial_minerals,_geology_and_man,_Northern_England

 See also CED 04 Red Whinstone