I had two Black Diamonds back in the day (late 70's or early 80's) Both were stolen.
Most of them were actually a metallic red colour with half chromed front forks - but there were some black ones too. Standard fittings included full chrome mudguards and a rear carrier. 10 speed gearing with down tube shifters.
They were made from standard hi tensile steel tubing and fitted with lower end components.... ..they were far from being light weight but they looked nice and rode quite nicely. Ernie Clements split from Falcon a year or two after these were made and set up his own Clements brand.
The Diamonds were quite popular and there are still a good few out there, they crop up frequently on
eBay and tend to go for £80 or so. Enjoy it.....and pictures please!