Where is the root morpheme in the Old English cristalla (crystal) and cymen (cumin)? It seems to be wrong to identify the morphemes in loanwords from etymological point of view.
There is an Old English word cristal,
Crystal: From Old English cristal (“clear ice/mineral”),
so I would assume that "cristalla" has the root of "cristal", and "-a" is a suffix.
From Old English cymen, from Latin cuminum,
The Latin cuminum is a second declension of "cumin", so the root in this case is the word itself, as it doesn't have any affix attached.