Find the odd material (gemstones vs rock): Select the item that is not a gemstone but a metamorphic rock.

Introduction / Context:
Minerals and rocks are distinct categories. Diamond, ruby, and sapphire are gemstones (mineral species or varieties). Marble is a metamorphic rock composed chiefly of recrystallized carbonate minerals, which places it in a different geological class.

Given Data / Assumptions:

• Diamond: gemstone (carbon crystal).
• Ruby: gemstone (corundum variety with chromium).
• Sapphire: gemstone (corundum variety with other trace elements).
• Marble: metamorphic rock, not a single mineral species.

Concept / Approach:
Classify by geology: single-mineral gemstones vs polymineralic (or rock-class) material. Marble’s composition and formation (metamorphism of limestone/dolostone) make it unlike the gemstones.

Step-by-Step Solution:

Verification / Alternative check:
Usage contexts: gemstones are cut for jewelry; marble is quarried for architecture/sculpture.

Why Other Options Are Wrong:
They belong to the gemstone class with distinct optical hardness and clarity properties.

Common Pitfalls:
Do not conflate “ornamental stone” with “gemstone”; gemology vs petrology categories differ.

Final Answer:
Marble