Classification (gemstones – typical appearance): Three gemstones are typically strongly coloured; one is characteristically colourless/near-colourless in its most valued form. Identify the odd one out.

Introduction / Context:
General-knowledge classification sometimes uses typical appearance to create a clean contrast. Ruby (red), emerald (green), and turquoise (blue-green) are prized for vivid body colour. By contrast, the benchmark ideal for diamonds is colourless (graded D–F as highest colour grades), with fancy colours treated as exceptions. The task is to identify the gemstone whose most valued common form is not defined by strong hue.

Given Data / Assumptions:

• Ruby → rich red (corundum, chromium colouring).
• Emerald → intense green (beryl, chromium/vanadium).
• Turquoise → blue to blue-green (hydrous phosphate mineral).
• Diamond → prized in a colourless/near-colourless state in mainstream grading.

Concept / Approach:
Group items by the criterion “valued primarily for vivid inherent hue.” The three coloured gemstones fit; diamond is typically evaluated for clarity, cut, carat, and absence of colour (except in fancy-colour categories). Hence diamond is the odd one under the typical valuation context used in reasoning questions.

Step-by-Step Solution:

Verification / Alternative check:
Ask which item would normally be described with a hue in everyday language (red ruby, green emerald, blue-green turquoise) versus “colourless diamond.”

Why Other Options Are Wrong:

Common Pitfalls:
Noting that “fancy-colour diamonds” exist; while true, the test focuses on typical valuation standards, where colourless is ideal.

Final Answer:
Diamond