Things that appear on the surface to be of great value may be quite worthless.
all that glitters is not gold
Something attractive is not always what it seems, as in This house is really beautiful, but a close look will show dry rot near the foundationall that glitters is not gold. Aesop stated the same idea in two of his fables (c. 600 b.c.), and a version close to the current wording appeared in 1175.