Hard rubber and Natural rubber

Hard rubber noun - A hard nonresilient rubber formed by vulcanizing natural rubber.

Natural rubber and hard rubber are semantically related. In some cases you can use "Natural rubber" instead a noun phrase "Hard rubber".

Natural rubber noun - An elastic material obtained from the latex sap of trees (especially trees of the genera Hevea and Ficus) that can be vulcanized and finished into a variety of products.

Hard rubber and natural rubber are semantically related. Sometimes you can replace phrase "Hard rubber" with "Natural rubber", this phrases are similar.