CGC Crystal Structure & Mineralogy 1 — Questions and Answers
Question 1: Which crystal system is characterized by three axes of equal length meeting at right angles?
- Hexagonal
- Cubic (Isometric) (Correct answer)
- Orthorhombic
- Tetragonal
Correct answer: Cubic (Isometric)
The cubic (isometric) system has three equal axes (a1, a2, a3) all at 90° to each other, producing highly symmetrical forms like the cube and octahedron.
Question 2: On the Mohs hardness scale, which gemstone has a hardness of 9?
- Topaz
- Diamond
- Corundum (Ruby/Sapphire) (Correct answer)
- Quartz
Correct answer: Corundum (Ruby/Sapphire)
Corundum, which includes ruby and sapphire, has a Mohs hardness of 9, second only to diamond (10) on the scale.
Question 3: What term describes the tendency of a mineral to break along flat planes related to its crystal structure?
- Fracture
- Cleavage (Correct answer)
- Parting
- Tenacity
Correct answer: Cleavage
Cleavage is the tendency of a mineral to break along smooth, flat planes that reflect the internal crystal structure and atomic bonding.
Question 4: Diamond belongs to which crystal system?
- Hexagonal
- Tetragonal
- Cubic (Isometric) (Correct answer)
- Trigonal
Correct answer: Cubic (Isometric)
Diamond crystallizes in the cubic (isometric) system, forming octahedral, cubic, and dodecahedral crystals due to the symmetric arrangement of carbon atoms.
Question 5: Which property describes the ratio of a gemstone's weight to the weight of an equal volume of water?
- Refractive Index
- Specific Gravity (Correct answer)
- Birefringence
- Luster
Correct answer: Specific Gravity
Specific gravity (SG) is the ratio of a mineral's density to that of water at 4°C, and it is a key diagnostic property for gemstone identification.
Question 6: Emerald is a variety of which mineral species?
- Corundum
- Tourmaline
- Beryl (Correct answer)
- Spinel
Correct answer: Beryl
Emerald is the green gem-quality variety of the mineral beryl (Be3Al2Si6O18), with its color caused by trace amounts of chromium and/or vanadium.
Question 7: What is the name for a crystal defect where a foreign atom replaces an atom in the crystal lattice, often causing color?
- Inclusion
- Twinning
- Isomorphous Substitution (Correct answer)
- Polymorphism
Correct answer: Isomorphous Substitution
Isomorphous substitution occurs when a foreign atom of similar size and charge replaces a host atom in the crystal lattice, a primary cause of color in many gemstones.
CGC Crystal Structure & Mineralogy 1
Which crystal system is characterized by three axes of equal length meeting at right angles?