Which of the following is NOT an igneous rock?

• A. Basalt
• B. Granite
• C. Obsidian
• D. Marble

Answer: (D)

Rocks are grouped by how they form. Igneous rocks come from molten material that cools and solidifies, either as lava at the surface or magma underground. Their crystals form as the melt cools. Basalt forms from rapidly cooled lava, giving a fine-grained texture. Granite forms from magma cooling slowly underground, yielding larger crystals. Obsidian is volcanic glass that solidifies so quickly it doesn’t have visible crystals.

Marble, on the other hand, is not formed by cooling molten rock. It starts as limestone and is transformed by heat and pressure deep underground, causing the calcite grains to recrystallize into a new, interlocking mosaic. This process is metamorphism, not igneous crystallization. So marble is a metamorphic rock, not igneous.