Origins
The hardest rock in the builder's arsenal is named for something as humble as a grain of wheat. Granite comes from Italian granito, the past participle of granire ('to granulate'), from Latin grānum ('grain, seed'). Italian stonemasons named it for what they saw: a stone made of visible grains.
The Latin grānum produced one of the wider word families in English. Grain itself is the direct descendant. A granary stores grain. Granules are small grains. A pomegranate is a pōmum grānātum — literally a 'seeded apple'. And the gemstone garnet takes its name from its resemblance to the deep red seeds inside a pomegranate.
Latin Roots
The word entered English through Italian rather than French or Latin because Renaissance Italy dominated European stonework. Italian quarries at Carrara and Baveno exported not just stone but vocabulary. When English speakers adopted granite in the 1640s, they borrowed the Italian masons' own term.
Geologically, granite forms deep underground when magma cools slowly enough for large crystals to develop — quartz, feldspar, mica, each visible to the naked eye. Those visible crystals are the 'grains' that gave the rock its name. A smooth stone would never have been called granito.