The research took longer than usual. Googled the wrong terms – pod, round ball pod, green pod with black spots, tree pod, gum tree pod, globe-like pod – finally “round green tree pod” led me to discover this isn’t your normal everyday seed pod. My first thought was that it was a protective shell around a young gum tree pod, after all, the interior DOES look like the gum tree “spur”. But no-o-o-o!! It’s an oak apple gall, and despite its look, it is NOT a seed (oak apple galls are so named because they resemble apples).
Galls are plant deformities caused by insects or a fungus. In this case, an oak apple gall wasp caused the oak leaf it grew on to develop into a sphere-shaped fiber mass in which a wasp larva had developed. There are several species of these wasps that create galls. Quite happy none were inside at the time I closed in with the macro lens (plus, I’m allergic to these particular insects).