I was asked to prove that the ring Z[sqrt[d]] is not euclidean for any d <= -3. (Note - The resulting ring it not contained in R!)

I proved it for certain d's easily (For example d=-3), by showing that it is not a UFD, but am not sure how to prove it in the more general case. (Or whether Z[sqrt[d]] is a UFD for some d's although it is not Euclidean)

Could anyone suggest a solution or intuition as to why is it not a Euclidean Domain?