Tower Black Diamond

[fido]

At some point in the '60s my mother got a Tower Black Diamond omelette pan with an insert for poaching 4 eggs. My grandmother had the same pan but without the poaching bits and I was given hers after she died, as I was setting up my first home of my own around that time. That was 1981 and until then I had assumed these pans were teflon coated but I also got a full size frying pan which WAS teflon coated and I soon came to realise there is a difference! Teflon is PTFE, quite a soft plastic, which comes off the pan quite easily if not handled with care. Black Diamond is a very hard coating which does eventually wear off but it takes about 20 years of regular scouring to wear through. I just wondered if anyone knows what material they used for Black Diamond?