....and what else, dazzlenation?...'mother of pearl' - slag rock, apparently and then we're back to inga and her so-called 'geography/geology' lessons, let us say.....go figure:

Found in London, Thames bankside : whatsthisrock (reddit.com)

Sold at Auction: Roman Glass Flask - Gorgeous Iridescence | Glass flask, Antique glass bottles, Roman glass jewelry (pinterest.co.uk)

hmmm....laura had had a look at the above, hadn't she?...and it is 'invaluable' - not 'slag':