Corsi Collection of Decorative Stones

view of stone 785

Corsi 785, approx. 145 x 73 x 40 mm, © Oxford University Museum of Natural History

OUMNH Number: 785  
Name and quarry location: Imperial porphyry, porfido bastardone, from Gebel Dokhan, Eastern Desert, Egypt  
Geological description: Andesite-dacite porphyry; the 'bastardone' variety with white phenocrysts of plagioclase feldspar largely concentrated in scattered patches of brown groundmass. The purple colour of the greater part of this stone is due largely to trace piemontite, the manganese member of the epidote group, and indicates a degree of metamorphism.  
Comments: Imperial porphyry comes from Gebel Dokhan (the ancient Mons Porphyrites) in the Eastern Desert of Egypt. This is the variety known as porfido bastardone. The geological term 'porphyry' for rocks with larger crystals in a fine-grained groundmass, comes from this rock which is itself named from the Latin for purple. The imperial quarries were worked from at least the 1st century through to the 5th century AD, using a workforce of slaves. The ancient quarries were rediscovered in 1823, and their location was described by the quarry hunter William Brindley (1887). They were never reopened on a commercial scale, so virtually every example of this stone seen around the world was quarried in ancient Roman times.  
References: Brindley (1887); Corsi (1845) 200-204; Harrell (2010a); Lazzarini (2002c) 233-235; Price (2007) 202-203  
Further information:

Corsi's classification: Class 15. Porphyry; Species 1.; i. Antique porphyry  
Corsi's text: 194.3 Porfido antico. Marmor Porphyrite. Porfirite. Porfido bastardone... la prime cave del porfido rosso si aprissero presso l'istmo di Sues. Di altra cave di porfido aperta nella Tebaide... si è chiamata Tebano. Marmo Romano  

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