You are in luck - the same item (possibly the same actual piece?), is illustrated on page 60 of
Sulphides The Art of Cameo Incrustation by Paul Jokelson, 1968. His text says:
A plaque by Martoret with the joined profiles of Voltaire and Rousseau, after a medal engraved by Caunois.
And the book also states that Martoret started in 1823 and he was a "manufacturer of crystal medals" in Paris.