Swear to Dog, it looked like some kind of fine-grained concrete to me. I can't see how they could possibly have built it any other way. The shell had no mold line at all, and it was TOUGH plastic, more like nylon or a hard urethane. The core had been poured in, I'd swear to it. I examined my "kill" in detail
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Now, this was an OLD worn-out bowling ball from a bowling alley so, it may not have been state of the art...