What is Jimmy's oval necklace?

[Johndpm]

I've been trying to make a replica of Jimmy's oval necklace, supposedly worn at Earls Court in 1975. The only image of it i can find when I search for it is the one attached below but it's a replica and I don't think it's really completely accurate. My main question is, does anyone know what the stone is and/or its dimensions? I'd also like to see if anyone has any better pictures of the actual necklace.

[Fall Of Day]

Hard to tell given stage lighting and 70s photography... iridescent sea shell, or blue-silver opal, or? 

Edited November 27, 2021 by Fall Of Day