Awhat can be the intermediate stage of speciation?

Speciation can be difficult to observe in  nature, but the it is hypothesized that the following occurs in a allopatric speciation's intermediate stage:

(1) closely related species are differentiated through geographic variation (2) then there's reduced fertility in one a geographic race of the species (3) and in the geographic race of subspecies that are to become species. 

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C. gemstones they are the only non metallic mineral from the list.