What is the material of which the transparent part of CD cases are made of? It is nicely rigid and really lightweight.
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These are called slimline jewel cases.
From the Wikipedia article on Optical Disc Packaging:
The bulk of slimline cases are made with translucent or transparent polystyrene, and are available in multiple colors. A stronger alternative is made from semi-opaque, semi-flexible polypropylene which is strong enough to protect the disc, but flexible enough not to break easily.
Here is an image of a polystyrene case to compare to yours: