For years, fans theorised that Diggle could actually be Green Lantern Corps member John Stewart (it was more wishful thinking than anything based in fact), and it's something Arrow started playing into.
It was eventually revealed that Diggle's stepfather was Roy Stewart, and during the Elseworlds crossover, the Earth-90 version of the Flash mentioned that the vigilante wasn't wearing his ring. By the time Arrow's series finale rolled around, The CW fully committed to that by having Diggle discover a box that crashed to Earth that, when opened, glowed green.
It containing a ring belonging to the Green Lantern Corps is a given, but actor David Ramsey has taken to his Instagram account to show fans what was really inside the box and...it...is...a glowing green cube.
In order to achieve the desired effect, a prop like that was essential, and it's not exactly surprising that there wasn't an actual Green Lantern ring in there. With Ramsey expected to have a recurring role in Superman & Lois, it's there we're expecting to see Diggle's superhero transformation.
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