Which igneous rock is known for its glassy appearance due to rapid cooling?

Obsidian is recognized for its glassy appearance, which results from the rapid cooling of molten lava. When lava cools quickly, it doesn't have enough time for crystal structures to form, leading to a smooth, glass-like texture. This characteristic is what sets obsidian apart from other igneous rocks.

In contrast, pumice, while also an igneous rock, features a vesicular texture due to gas bubbles trapped during its formation, rather than a glassy appearance. Basalt is a common, fine-grained igneous rock formed from the slow cooling of lava, which allows for some crystallization, while granite is a coarse-grained igneous rock that typically forms from the slow cooling of magma deep within the Earth's crust, leading to well-formed crystals. Therefore, obsidian is distinctly identified by its glassy appearance resulting from its formation process.