They probably were made out of resin.I am not sure of the correct term for this clock material, but they were commonly referred to as Adamantine mantle clocks. I have not seen frequent cracking of these clocks which often came from common homes of late 19th to the early 20th Century. Many of the clocks in the below picture are of this make in my collection and also one closeup.
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Back in the late 1890s, resin was used quite a bit as grip panels and gun furnitures with all sorts of wild and crazy colors and patterns.