Zoroastrianism was the original religion of Iran, and the oldest monotheism (belief in one God) religion in the world. Now practiced by about 190,000 people, there are a number of important sites in or near Yazd.
Towers of Silence
Place of the famous Zoroastrian death ritual where dead bodies were left for vultures to pick clean to the bone. It was abandoned when the vultures started to be replaced by crows, who dropped what they found around town.
There are two towers, both worth the effort to climb up, to see the place where bodies were left, and for views over the complex (including the replacement cemetery where bodies are buried in concrete lined graves to avoid contact with the earth), and the town.
Atashkadeh Fire Temple
The fire contained within has been burning continuously since 470AD, but other than this the temple is fairly underwhelming. Zoroastrians are not ones for highly decorative temples, refreshing in a way from many religions.