The Aon Center (previously the "Standard Oil Building") was originally built with a thin marble exterior. Some marble fell, and other cracked, so about 20 years later, its exterior was replaced entirely with airy granite, costing about $80 million.

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The building, completed in 1973, was the tallest in the building in Chicago and the fourth in the world. The original design called for 43,000 marble slabs, and, during construction, a 350lbs slab fell off.

Years later, cracks were found and were supported using steel straps.

Then, from 1990-1992, the marble was replace in full with airy granite.

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