Chemist’s artificial gems, usually rubies at first (7)
I believe the answer is:
pasteur
'chemist's' is the definition.
(Pasteur is an example)
'artificial gems usually rubies at first' is the wordplay.
'artificial gems' becomes 'paste' (I've seen this in another clue).
'at first' says to take the initial letters.
The first letters of 'usually rubies' is 'ur'.
'paste'+'ur'='PASTEUR'
(Other definitions for pasteur that I've seen before include "He developed the partial sterilization of milk" , "Louis ---, Fr. bacteriologist" , "Louis -, nineteenth century French chemist" , "French chemist and biologist, d. 1895" , "Pioneer immunologist" .)