Made entirely of fired brick, the minaret is composed of a two-tier cylindrical body raised seven meters on an octagonal base. Two spiral staircases, accessed from a single doorway above the ground, provide access to two balconies atop the lower shaft and midway up the upper shaft, visiting six vaulted chambers located in between. 

The minaret in Jam, Afghanistan (12th century)
The minaret in Jam, Afghanistan (12th century) © Johannes Hertz

The two towers of the minaret shaft differ structurally. The broad lower shaft is made of thick walls enveloping two spiral staircases at center; it rises to a height of thirty-eight meters, with an exterior diameter diminishing from 9.7 meters at the base to 6 meters at top. The narrow upper shaft, by contrast, has a central void spanned by six cross-vaults resting on four internal buttresses. The stairs are here channeled into the narrow spaces between the walls and the buttresses.

The rich decoration of the minaret, executed with tiles and terracotta in high relief, has largely survived and was analyzed in detail by Sourdel-Thomine. The upper shaft features three epigraphic bands -- one below the cupola and two below the second balcony -- that contain the shahada, Quranic verses fromSura al-Saff and the name of Ghiyath al-Din, respectively. The two lower bands are separated by a thick decorative band featuring a symbolic vase motif found in the Ghazna palace of Mas'ud III and on coins issued at Firuzkuh.