Gemology & Mineralogy Fundamentals Flashcards

In mineralogy, 'twinning' in diamond most commonly produces which crystal form?

Answer: Macle (flattened triangular twin)

Macles are flattened triangular contact twins common in diamond, where two crystals share a twin plane along an octahedral face.

What is the primary geological source rock for natural diamonds?

Answer: Kimberlite

Kimberlite pipes are the primary economic source of diamonds, originating from deep mantle magmas that rapidly ascend to the surface.

Which element is responsible for the yellow color in most Type Ib diamonds?

Answer: Isolated nitrogen atoms

Type Ib diamonds contain isolated (dispersed) nitrogen atoms that absorb blue light, causing the stone to appear yellow or orange.

The term 'naats' or 'natts' in diamond trade refers to:

Answer: Structural grain directions related to cleavage

Naats (or natts) describe the directional grain of a diamond related to its atomic planes, which must be considered during cutting and polishing.

What distinguishes a 'lamination' inclusion in a diamond?

Answer: A flat, reflective platelet parallel to crystal planes

Laminations are flat, reflective inclusions (often nitrogen platelets) that lie parallel to the octahedral crystal planes within a diamond.

Which optical phenomenon causes a 'Graining' feature in a diamond?

Answer: Irregular crystal growth or twinning producing internal reflection

Graining results from irregular crystal growth or twinning, creating internal planes that reflect light differently and are visible as lines or ripples.

What is the approximate depth at which natural diamonds form in the Earth?

Answer: 80–200 km (upper mantle)

Most gem-quality diamonds crystallize in the upper mantle at depths of approximately 80–200 km, where pressure and temperature conditions are suitable.