Here's why:

* Composition: Both basalt and gabbro are igneous rocks composed primarily of plagioclase feldspar and pyroxene minerals.

* Cooling Rate: The difference lies in their cooling rate. Gabbro cools slowly underground, allowing for larger crystals to form. Basalt cools rapidly, typically at or near the surface, leading to smaller, less visible crystals.

Therefore, they share the same mineral composition but have distinct grain sizes due to different cooling conditions.