Lesson Notes By Weeks and Term v3 - Senior Secondary 3
Identification of Gemstones
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Identification of Gemstones
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Subject: Mining
Class: Senior Secondary 3
Term: 2nd Term
Week: 3
Theme: Gemology And Lapidary
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Mention the effective methods of gemstone identification. Identify common varieties of gemstones in hand specimen and under the microscope.
breaks when it does not follow a cleavage plane. Types include conchoidal (shell-like, e.g., quartz, glass), uneven, hackly (jagged, e.g., copper).
Observation: Observe broken surfaces for signs of cleavage planes.
4. Tenacity: How a mineral responds to breaking, crushing, bending, or tearing (e.g., brittle, malleable, ductile, sectile, elastic). Most gemstones are brittle.
C. Optical Properties (Microscopic/Gemological Instrument Level): These methods require specialized instruments and provide highly accurate results.
1. Refractive Index (RI): A measure of how much light bends when it enters a gemstone from the air. Each gemstone has a characteristic RI (or range).
Instrument: Refractometer.
Concept: A beam of light enters the gemstone, and the degree of bending is measured.
Example: Ruby and Sapphire (Corundum) have an RI of 1.76-1.
7
7. Quartz has an RI of 1.54-1.55.
2. Birefringence (Double Refraction): The difference between the maximum and minimum refractive indices in an anisotropic (doubly refractive) gemstone. Light splits into two rays as it passes through.
Instrument: Polariscope, or observing facet junctions through a microscope.
Concept: When a gemstone is rotated between polarized filters, it may show extinction (darkening) or light-dark patterns. Strong birefringence can cause "doubling" of facet edges when viewed through a loupe or microscope (e.g., zircon, peridot).
3. Dispersion ("Fire"): The splitting of white light into its spectral colours (rainbow effect) as it passes through a gemstone. High dispersion creates more "fire." Observation: Visually observing the flashes of colour, especially in well-cut gems.
Example: Diamond has very high dispersion, giving it its characteristic "fire."
4. Pleochroism: The phenomenon where a gemstone appears to have different colours when viewed from different crystallographic directions.
Instrument: Dichroscope, or viewing through a polariscope.
Example: Tourmaline often shows strong pleochroism, appearing different shades of green or pink depending on the viewing angle.
5. Spectroscopy: Analyzing the unique absorption and emission spectra of light as it passes through a gemstone.
Instrument: Spectroscope.
Concept: Certain elements absorb specific wavelengths of light, creating dark lines or bands in the spectrum, which act as a fingerprint for the gem.
6. Microscopic Examination (Inclusions): Internal features (inclusions) within a gemstone are often characteristic and can indicate natural origin, specific localities, or treatment.
Instrument: Gemological microscope.
Observations: Gas bubbles (common in glass or synthetic gems), mineral crystals, fluid inclusions ("fingerprints"), growth zoning, cleavage planes, fractures.
Example: Natural emerald often contains three-phase inclusions (solid, liquid, gas), while synthetic emerald might contain curved growth lines.
D. Other Methods:
1. Fluorescence: The emission of light by a gemstone when exposed to ultraviolet (UV) radiation.
Test: Using long-wave and short-wave UV lamps.
Example: Some diamonds fluoresce blue under UV light; synthetic rubies might fluoresce a strong red.
2. Thermal Conductivity: How well a gemstone conducts heat.
Instrument: Thermal conductivity tester (e.g., diamond tester).
Example: Diamond is an excellent thermal conductor, while most simulants are poor conductors, allowing testers to differentiate them. Common Varieties of Gemstones (Hand Specimen and Microscopic Features):
1. Quartz (SiO2): Hardness: 7 SG: 2.65 Fracture: Conchoidal Varieties: Amethyst (purple, often with colour zoning), Citrine (yellow-orange), Smoky Quartz (brown/grey), Rose Quartz (pink), Rock Crystal (colourless).
Identification: Presence of growth lines, twinning, and specific inclusions (e.g., rutile needles in rutilated quartz). Common in Nigeria (Plateau, Taraba).
2. Corundum (Al2O3): Hardness: 9 (second hardest natural mineral) SG: 3.96-4.05 RI: 1.76-1.77 (Double Refractive, uniaxial)
Varieties: Ruby (red, coloured by chromium), Sapphire (all other colours, typically blue from iron and titanium).
Identification: High hardness, strong pleochroism (especially blue sapphire), straight growth zoning, angular inclusions (rutile needles forming "silk" in star gems). Found in Kaduna, Plateau, Taraba.
3. Beryl (Be3Al2Si6O18): Hardness: 7.5-8 SG: 2.65-2.90 RI: 1.56-1.60 Varieties: Emerald (green, coloured by chromium/vanadium), Aquamarine (blue-green, coloured by iron), Heliodor (golden yellow), Morganite (pink).
Identification: Hexagonal crystal habit, characteristic inclusions like "jardin" (garden-like) in emeralds (3-phase inclusions). Emeralds often treated with oil. Found in Kaduna, Nasarawa.
4. Tourmaline (Complex Borosilicate): Hardness: 7-7.5 SG: 3.0-3.26 RI: 1.61-1.65 (Strongly double refractive)
Varieties: Wide range of colours (Rubellite-pink/red, Indicolite-blue, Verdelite-green). Often exhibits striking colour Definition of a Gemstone: A gemstone is a piece of mineral crystal, which, in cut and polished form, is used to make jewelry or other adornments. Certain rocks (such as lapis lazuli and opal) and organic materials (such as amber and pearl) are also sometimes classified as gemstones. Key characteristics that define a gemstone include:
1. Beauty: Attractiveness, often due to colour, brilliance, fire, and optical effects.
2. Durability: Resistance to scratching (hardness), breaking (toughness), and chemical alteration.
3. Rarity: Not commonly found, contributing to its value.
Why Gemstone Identification is Crucial: Accurate identification is vital for: Valuation: Different gemstones have vastly different market values.
Authentication: Distinguishing natural gems from synthetic, imitation, or treated materials.
Classification: Correctly categorizing minerals for geological and commercial purposes.
Fraud Prevention: Protecting buyers and sellers from misrepresentation. Effective Methods of Gemstone Identification:
A. Visual Examination (Macroscopic/Hand Specimen Level): This is the initial step and involves using the naked eye or a hand lens (loupe) to observe distinct features.
1. Colour: The most obvious feature, but often unreliable for definitive identification as many gems share similar colours (e.g., red ruby vs. red garnet). Hue, tone, and saturation are important. Some gems are idiochromatic (colour due to essential element, e.g., peridot is always green due to iron) while others are allochromatic (colour due to impurities, e.g., quartz can be many colours).
2. Transparency: Describes the degree to which light passes through the gemstone (transparent, translucent, opaque).
3. Luster: The way light reflects off the surface.
Vitreous (glassy): Most common (e.g., quartz, topaz).
Adamantine (diamond-like): Highly reflective (e.g., diamond, zircon).
Resinous (resin-like): (e.g., amber).
Pearly: (e.g., pearl, moonstone).
Silky: (e.g., tiger's eye).
4. Optical Phenomena: Special light effects.
Play of Colour (Iridescence): Rainbow-like flashes (e.g., opal).
Chatoyancy (Cat's Eye Effect): A single bright band of light across the surface (e.g., chrysoberyl cat's eye, tiger's eye).
Asterism (Star Effect): A star-shaped pattern of light (e.g., star sapphire, star ruby), caused by reflections from tiny needle-like inclusions.
Adularescence: A bluish-white sheen (e.g., moonstone).
B. Physical Properties (Hand Specimen/Simple Tool Level): These properties require simple tests and provide more definitive identification criteria.
1. Hardness (Mohs Scale): Resistance to scratching. Mohs scale ranges from 1 (softest, e.g., Talc) to 10 (hardest, e.g., Diamond).
Practical Test: Scratching the unknown gemstone with materials of known hardness.
Fingernail: Hardness ~2.5 Copper coin (1 Kobo/2 Kobo/5 Kobo if available, or brass key): Hardness ~3.0-3.5 Steel knife/nail: Hardness ~5.0-5.5 Glass plate: Hardness ~5.5-6.0 Steel file: Hardness ~6.5 Quartz crystal: Hardness 7
Example: To differentiate quartz (H=7) from a glass imitation (H=5.5), try scratching the unknown sample with a steel knife. If it scratches, it's likely glass; if it doesn't, it could be quartz or harder.
2. Specific Gravity (SG): The ratio of the density of a gemstone to the density of an equal volume of water at 4°
C. It's a measure of how heavy a gem feels for its size.
Measurement Concept: Usually measured using hydrostatic weighing (weighing the gem in air and then suspended in water). The greater the SG, the heavier the gem will feel.
Example: Diamond has an SG of ~3.52, while cubic zirconia (a common diamond simulant) has an SG of ~5.6-5.
9. A diamond would feel lighter than a CZ of the same size.
3. Cleavage and Fracture: Cleavage: The tendency of a mineral to break along smooth, flat planes of structural weakness (e.g., mica has perfect cleavage, topaz has perfect basal cleavage).
Fracture: The way a mineral breaks when it does not follow a cleavage plane. Types include conchoidal (shell-like, e.g., quartz, glass), uneven, hackly (jagged, e.g., copper).
Observation: Observe broken surfaces for signs of cleavage planes.
4. Tenacity: How a mineral responds to breaking, crushing, bending, or tearing (e.g., brittle, malleable, ductile, sectile, elastic). Most gemstones are brittle.
C. Optical Properties (Microscopic/Gemological Instrument Level): These methods require specialized instruments and provide highly accurate results.
1. Refractive Index (RI): A measure of how much light bends when it enters a gemstone from the air. Each gemstone has a needles forming "silk" in star gems). Found in Kaduna, Plateau, Taraba.
3. Beryl (Be3Al2Si6O18): Hardness: 7.5-8 SG: 2.65-2.90 RI: 1.56-1.60 Varieties: Emerald (green, coloured by chromium/vanadium), Aquamarine (blue-green, coloured by iron), Heliodor (golden yellow), Morganite (pink).
Identification: Hexagonal crystal habit, characteristic inclusions like "jardin" (garden-like) in emeralds (3-phase inclusions). Emeralds often treated with oil. Found in Kaduna, Nasarawa.
4. Tourmaline (Complex Borosilicate): Hardness: 7-7.5 SG: 3.0-3.26 RI: 1.61-1.65 (Strongly double refractive)
Varieties: Wide range of colours (Rubellite-pink/red, Indicolite-blue, Verdelite-green). Often exhibits striking colour zoning.
Identification: Strong pleochroism, strong double refraction (can see "doubling" of facet edges), distinct triangular cross-section of crystals, often with parallel growth striations. Notably found in Oyo, Kaduna, Plateau, Kwara.
5. Garnet (Complex Silicates): Hardness: 6.5-7.5 (depending on variety) SG: 3.5-4.3 RI: 1.71-1.89 (Single Refractive, isotropic)
Varieties: Almandine (red-brown), Pyrope (deep red), Spessartine (orange-red), Grossular (green, yellow, orange), Demantoid (green).
Identification: Isotropic (no double refraction), often found in dodecahedral or trapezohedral crystal forms. "Horsetail" inclusions are characteristic of Demantoid. Found in Kaduna, Plateau.
6. Topaz (Al2SiO4(F,OH)2): Hardness: 8 SG: 3.49-3.57 RI: 1.60-1.63 Identification: Orthorhombic crystals, perfect basal cleavage (must be handled with care), often colourless, blue, yellow, or pink. Found in Plateau.
7. Zircon (ZrSiO4): Hardness: 6.5-7.5 SG: 3.90-4.70 (variable due to metamictization) RI: 1.81-1.98 (very high, strong double refraction)
Identification: Distinctive strong double refraction ("doubling" of back facet edges), high luster and fire. Often occurs as brown-red, yellow, or green. Used as a diamond simulant due to high dispersion. Found in Kaduna, Plateau. Worked
Example: Differentiating between a real gemstone and an imitation A student finds a beautiful red stone in a local market in Jos, claiming it to be a valuable ruby. How can simple tests help to determine if it is indeed a ruby or a less valuable simulant like red glass or garnet?
Step-by-step Reasoning:
1. Visual Examination: Colour: Observe the shade of red. Rubies are typically a vibrant pigeon-blood red. Red glass and garnet also come in various shades of red. This alone is not conclusive.
Luster: Rubies have a vitreous to sub-adamantine luster. Glass has a vitreous luster. Garnet (Pyrope/Almandine) typically has a vitreous luster. This is still not definitive.
Clarity: Look for internal flaws or inclusions. Natural rubies almost always have some inclusions (e.g., "silk" rutile needles, crystal inclusions). Red glass might have spherical bubbles or swirl marks. Garnets also have inclusions.
2. Hardness Test: Hypothesis: Ruby has a hardness of
9. Garnet 6.5-7.
5. Glass ~5.5-6.
0. Procedure: Try to scratch the stone with a known quartz crystal (H=7). If the quartz scratches the stone, it is definitely not a ruby (and likely glass or a softer garnet). If the quartz does not scratch the stone, it could be a ruby or a very hard garnet (though less likely to be scratched by quartz).
Further Test: If available, try to scratch a piece of known topaz (H=8) with the stone. If the stone scratches topaz, it has a hardness >8, further suggesting it could be ruby.
Crucial point: Always test an inconspicuous area and ensure the item being scratched is not the precious stone itself, but rather the tester.
3. Specific Gravity Test (Conceptually): If a specific gravity liquid set is available, or hydrostatic weighing can be performed, measure the SG. Ruby (Corundum) has an SG of ~4.
0. Red Garnets have SGs typically between 3.5 and 4.
3. Glass has an SG of ~2.4-2.
8. If the measured SG is significantly lower than 3.5, it is likely glass. If it falls within the garnet range, it's more likely garnet. If it is around 4.0, it could be ruby or a heavy garnet. This helps narrow down possibilities.
4. Microscopic Examination (if available): Examine internal inclusions for bubbles (glass), characteristic "silk" (rutile needles) or growth patterns (ruby), or other crystal inclusions (garnet). * Check for doubling of facet edges. Ruby (corundum) is doubly refractive and will show subtle doubling, while garnet is singly refractive and will not. Glass is also SG is significantly lower than 3.5, it is likely glass. If it falls within the garnet range, it's more likely garnet. If it is around 4.0, it could be ruby or a heavy garnet. This helps narrow down possibilities.
4. Microscopic Examination (if available): Examine internal inclusions for bubbles (glass), characteristic "silk" (rutile needles) or growth patterns (ruby), or other crystal inclusions (garnet). Check for doubling of facet edges. Ruby (corundum) is doubly refractive and will show subtle doubling, while garnet is singly refractive and will not. Glass is also singly refractive.
Conclusion: By combining several tests, especially hardness and observation of internal features, one can significantly narrow down the identity. If the stone is hard (scratches quartz), shows no spherical bubbles, and exhibits characteristics of double refraction or natural inclusions, it is more likely to be a genuine ruby (or another hard, natural gemstone). If it is easily scratched by quartz and contains bubbles, it is likely glass.
Economic Development and Entrepreneurship: Gemstone identification is critical for artisanal miners and small-scale traders in regions like Plateau, Kaduna, and Oyo states. Correctly identifying a valuable gem like tourmaline or sapphire helps them demand fair prices, reducing exploitation by middlemen and fostering local entrepreneurship in the gem trade. This knowledge can transform raw finds into economic opportunities, supporting local livelihoods. Consumer Protection and Anti-Fraud Measures: In bustling Nigerian markets, especially those selling jewelry (e.g., Balogun Market in Lagos), the ability to distinguish genuine gemstones from common imitations (like glass or synthetic materials) protects consumers from fraud. It empowers individuals, including buyers and sellers, to make informed decisions, ensuring transparency and trust in transactions involving precious stones.
Resource Mapping and Geological Survey: Geoscientists and prospectors use gemstone identification skills to accurately map and assess Nigeria's mineral resources. Knowing the specific types of gemstones present in a deposit aids in geological surveys, informing mining investment decisions, and contributing to the national inventory of solid minerals for sustainable exploitation and management. For instance, identifying specific varieties of beryl or corundum indicates certain geological environments, guiding further exploration efforts.