Hexagonal Crystal System Gemstones: Emerald, Aquamarine and Beryl

The hexagonal crystal system is home to some of the most beloved gemstones in the world. Emerald, aquamarine, morganite, and all other beryl varieties crystallize in this system, along with apatite and several other important minerals. The six-fold symmetry of hexagonal crystals produces distinctive prismatic shapes and fascinating optical properties that gemologists and collectors prize highly.

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What Is the Hexagonal Crystal System?

The hexagonal system is defined by four crystallographic axes: three equal horizontal axes (a1, a2, a3) intersecting at 120 degrees, and one vertical c-axis perpendicular to them. This geometry creates a 6-fold symmetry axis, meaning the crystal looks identical when rotated 60 degrees around the c-axis.

Key Characteristics of Hexagonal Gems

• Doubly refractive - light splits into two rays inside the crystal
• Dichroism possible - may show two different colors from different directions
• Crystal habit: six-sided prisms with flat or pyramidal terminations
• Uniaxial optical character - one optic axis along the c-axis

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Beryl: The Hexagonal Gem Family

Beryl (beryllium aluminum silicate) is the most important hexagonal gem mineral. Pure beryl is colorless, but trace elements create a stunning range of gem varieties, all sharing the same hexagonal crystal structure.

Variety	Color	Cause of Color
Emerald	Green	Chromium and vanadium
Aquamarine	Blue to blue-green	Iron (Fe2+)
Morganite	Pink to peach	Manganese
Heliodor	Yellow to golden	Iron (Fe3+)
Goshenite	Colorless	No trace elements
Red Beryl	Red	Manganese
Maxixe	Deep blue	Color centers

All beryl varieties form as six-sided prismatic crystals, often with flat basal terminations. Single crystals can reach enormous sizes - the largest gem-quality aquamarine crystal ever found weighed over 110 kg.

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Emerald: The King of Hexagonal Gems

Emerald is the green variety of beryl, colored by chromium and sometimes vanadium. It is one of the four precious gemstones alongside diamond, ruby, and sapphire, and commands some of the highest prices per carat in the gem world.

Emerald's Hexagonal Properties

• Refractive index: 1.577 to 1.583
• Birefringence: 0.006 (low, making doubling hard to see)
• Pleochroism: Weak to moderate, bluish green to yellowish green
• Crystal habit: Six-sided prisms, often with natural etch marks on prism faces

Emeralds are famous for their jardin (French for garden) - the characteristic inclusions of fractures, crystals, and growth tubes that are accepted and even expected in fine emeralds. Eye-clean emeralds are extremely rare and command exceptional premiums.

Major Emerald Sources

• Colombia: Muzo, Chivor, and Coscuez mines produce the world's finest emeralds
• Zambia: Kafubu mining area produces deeply saturated stones
• Brazil: Large production, often lighter in tone
• Zimbabwe: Sandawana mine produces small but intensely colored stones

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Aquamarine: The Sea-Blue Hexagonal Gem

Aquamarine is the blue to blue-green variety of beryl, colored by iron. Its name comes from the Latin for seawater, perfectly describing its characteristic color. Unlike emerald, aquamarine typically forms large, clean crystals with few inclusions.

Aquamarine's Key Properties

• Refractive index: 1.577 to 1.583
• Color range: Pale blue to deep blue-green; heat treatment removes green tones
• Clarity: Usually eye-clean; inclusions are uncommon
• Crystal size: Can form very large crystals suitable for big faceted stones

The most prized aquamarines are a pure, deep blue without green overtones. The famous Santa Maria color, named after a mine in Brazil, describes the finest deep blue aquamarines.

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Other Important Hexagonal Gemstones

Apatite

Apatite crystallizes in the hexagonal system and occurs in a wide range of colors including neon blue-green, yellow, and purple. It is relatively soft at Mohs 5, making it better suited for collectors than everyday jewelry.

Sugilite

This rare purple cyclosilicate crystallizes in the hexagonal system and is prized for its vivid purple to violet color.

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Optical Properties of Hexagonal Gems

All hexagonal gemstones are uniaxial and doubly refractive. This means light entering the crystal splits into two rays traveling at different speeds. Gemologists use this property to help identify hexagonal gems:

• Refractometer: Shows two RI readings (ordinary and extraordinary rays)
• Polariscope: Shows anisotropic behavior (blinks on and off when rotated)
• Dichroscope: Reveals pleochroism - different colors visible from different crystal directions

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Frequently Asked Questions

Why do all beryl varieties have the same crystal structure but different colors?

All beryl varieties share the same hexagonal beryllium aluminum silicate structure. Color differences arise from different trace elements substituting into the crystal lattice. Chromium creates green (emerald), iron creates blue (aquamarine), and manganese creates pink (morganite).

How can I tell emerald from green tourmaline?

Both are green and doubly refractive, but tourmaline is trigonal, not hexagonal. Key differences include refractive index (emerald 1.577-1.583 vs tourmaline 1.624-1.644), specific gravity, and absorption spectrum visible under a spectroscope.

Do hexagonal gems show birefringence like zircon?

Hexagonal gems are doubly refractive but most have low birefringence. Beryl varieties have birefringence of only 0.006, making facet doubling nearly invisible. Zircon (tetragonal) has much higher birefringence of 0.059, making doubling clearly visible.

What is the largest emerald ever found?

The Bahia Emerald, found in Brazil in 2001, is one of the largest emerald specimens ever discovered, containing an estimated 180,000 carats of emerald crystals embedded in host rock.

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Conclusion

The hexagonal crystal system gives us some of the most treasured gems in human history. From the lush green of Colombian emerald to the serene blue of Santa Maria aquamarine, the six-fold symmetry of hexagonal crystals produces stones of extraordinary beauty. Understanding the hexagonal system helps you appreciate not just how these gems look, but why they form the way they do and how gemologists identify them.