Portugese tiles, called azulejos, adorn Lisbon's buildings. You can easily spend your entire trip there walking around and admiring these handmade, decorative tiles. Gaping holes on the crumbling walls of many buildings around Lisbon are as a result of thieves yanking them off to sell the tiles on the black market for a lot of money, especially if they belong to important and historic buildings.
If you buy azulejos from street vendors, check the back of the tiles to make sure you are not supporting stolen art.
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From @fjorenca