A family noticed that the decorative rocks supporting their patio have begun to turn red. What has most likely led to this discoloration and why is this a problem?

Rusting of iron-rich mineral content in the rocks is the reason for red discoloration of the decorative rocks.

Explanation:

A patio is an outdoor area that is usually decoratively floored with sandstone paving. Sandstone is a rock which contains high amount of iron oxides. Oxidation of these iron oxides due to the wet floors of patio could lead to rust formation or red color of these decorative rocks. Discoloration could also take place due to the long presence of rusted metallic objects and wet patio floor.

This is a problem because it causes stains on the patio paving which will spoil the decorative ambience. This could lead to corrosion and de-stability of the rock leading to chipping or cracking.