From the books of the Wizard of Oz:
<warning - spoilers!>
For those that have read it, recall how it was a requirement to wear lockable (as in locked-and-needing-a-key-to-remove) glasses that were tinted green, to give the city the appearance of emeralds, while disguising the fact it was simply tinsel, glass, and foil.
"11. The Wonderful City of Oz
Even with eyes protected by the green spectacles, Dorothy and her
friends were at first dazzled by the brilliancy of the wonderful City.
The streets were lined with beautiful houses all built of green marble
and studded everywhere with sparkling emeralds. They walked over a
pavement of the same green marble, and where the blocks were joined
together were rows of emeralds, set closely, and glittering in the
brightness of the sun. The window panes were of green glass; even the
sky above the City had a green tint, and the rays of the sun were green."
The citizens were forbidden to remove them, lest they see that the city was a sham.