According to The Oxford English Dictionary, John Gower, the 14th century poet, was first to use the comparison in English in his work, 'Confessio Amantis'. There, he wrote the sentence: "Lo, how they feignen chalk for chese". (They spelt oddly in those days!) Since then, it has been used in various proverb situations because, although the two materials may LOOK alike, they are very different. The fact that they both start with the 'ch' sound also helps to make the comparison memorable. (Gower appears to have had no connection whatever with Wiltshire!)