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Introduction to Crystal

Figure 1: Crystal (Citrine)

Quartz is one of the most abundant rock-forming minerals in the Earth’s crust and is widely used in the jewelry industry. Monocrystalline quartz is commonly referred to as crystal. It is a colorless to transparent gemstone that is highly valued for its clarity and brilliance.

• Chemical composition: Silicon dioxide (SiO₂)

• Color variations: Pure quartz is colorless, but trace elements such as aluminum and iron can form color centers after irradiation, producing colors like smoky, purple, yellow, and more.

Crystals are highly versatile and are popular in jewelry, decorative objects, and mineral collections.

Crystal Identification

A gemstone can be confirmed as crystal only if it meets all the following identification characteristics.

1. Refractive Index and Birefringence

Use a gem refractometer to measure the refractive index and birefringence (Figure 2).

• Refractive index: 1.544 – 1.553

• Birefringence: 0.009

Figure 2: Gem refractometer

Figure 3: Refractive index reading

2. Optical Characteristics

The gem polariscope is used to detect optical properties (Figure 4). Crystal can be reliably identified because it exhibits a bull’s eye interference figure, a unique feature not found in other gemstones.

• Optical properties:

  • Anisotropic (shows four bright and four dark sectors under a polariscope)

  • Uniaxial crystal

  • Bull’s eye interference figure (Figure 5)

  • Positive optic sign (measured with a refractometer)

Figure 4: Gem polariscope

Figure 5: Uniaxial crystal interference figure

3. Pleochroism

Pleochroism depends on the crystal’s color and is measured using a dichroscope (Figure 6).

• White crystal: weak pleochroism (Figure 7)

• Colored crystals: the strength and color change of pleochroism depend on the crystal color and depth

Figure 6: Dichroscope

Figure 7: Pleochroism of crystal

4. Density

Use a density balance to measure the density of crystal.

• Density: 2.66 g/cm³ (relatively stable)

Figure 8: Density balance

5. Microscopic Observation

Magnified features of crystal can be observed using a gemological microscope (Figure 9).

Purchase Gemological Microscope

• Natural crystals: contain abundant internal inclusions, such as:
  

  

  • Needle-like inclusions (Figure 10)
    

    

  • Sheet-like mineral inclusions (Figure 11)
    

    

  • Facet edge double images (Figure 12)
    

    

  • Gas-liquid inclusions (Figure 13)
    

    

  • Mist-like and fine needle inclusions (Figure 14)
    

    

  • Green needle-shaped mineral inclusions (Figure 15)
    

    

  • Dendritic inclusions (Figure 16)
    

    

  • Worm-like inclusions (Figure 17)
    

    

  • Oil inclusions (Figure 18)
    

    

  • Hair-like filamentous inclusions (Figure 19)

Figures 10–19: Natural crystal internal features

• Synthetic crystals: often show irregular “bread-crumb” like inclusions (Figures 20–21)

• Treated crystals:
  

  

  
  

  • Dyed: color concentrated along cracks (Figure 22)
    

    

  • Coated: surface coating visible under magnification (Figure 23)

Figures 20–23: Synthetic and treated crystal features

✅ Key Takeaways

• Crystal identification relies on refractive index, optical properties, pleochroism, density, and microscopic observation.

• Natural crystals have characteristic internal inclusions, while synthetic and treated crystals display different patterns, coatings, or concentrated dyes.

• Combining multiple testing methods ensures accurate identification.

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