The Mathura school was a homegrown style: it carved the Buddha in a robust, fleshy, smiling form modelled on the earlier indigenous Yaksha figures, using the local spotted red sandstone of Sikri.

The Gandhara school, flourishing in the north-west under Indo-Greek and Kushana contact, gave the Buddha wavy hair, sharp features and toga-like drapery — clearly Hellenistic or Greco-Roman in feel — and typically used grey schist/phyllite.

So the genuine difference is the indigenous Yaksha-inspired Mathura imagery versus the Hellenistic-influenced Gandhara imagery. The claim that "Mathura art primarily used green phyllite, while Gandhara art used spotted red sandstone" reverses the stone types, "Gandhara art developed on purely indigenous lines, whereas Mathura art showed foreign influences" reverses which school had foreign influence, and "Mathura art predominantly focused on Buddhist themes, while Gandhara focused on Hindu deities" wrongly assigns Hindu deities to Gandhara; all three misstate the facts.

Hence, the answer is that Mathura art depicted the Buddha in an indigenous Yaksha-like style whereas Gandhara art reflected Hellenistic (Greco-Roman) features.