Trigonal Crystal System Gemstones: Ruby, Sapphire and Quartz

The trigonal crystal system is the most gem-rich of all seven crystal systems. Ruby, sapphire, all quartz varieties, tourmaline, and rhodochrosite all belong to this system. Despite sharing a structural framework with the hexagonal system, trigonal crystals have their own distinct 3-fold symmetry that produces unique optical and physical properties.

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What Is the Trigonal Crystal System?

The trigonal system shares the same four-axis framework as the hexagonal system but has a 3-fold symmetry axis instead of 6-fold. This means the crystal looks identical when rotated 120 degrees around its main axis, rather than 60 degrees.

Like hexagonal gems, trigonal gems are uniaxial and doubly refractive. Light entering the crystal splits into two rays, which is a key diagnostic property in gemological identification.

Key Characteristics of Trigonal Gems

• Doubly refractive - uniaxial optical character
• Pleochroism common - different colors visible from different directions
• Crystal habits: rhombohedra, scalenohedra, six-sided prisms
• Often shows strong color saturation due to chromophore interactions with the lattice

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Corundum: Ruby and Sapphire

Corundum (aluminum oxide) is the most important trigonal gem mineral. Pure corundum is colorless, but trace elements create two of the world's most prized gemstones: ruby and sapphire.

Ruby: The Red Trigonal Gem

Ruby is red corundum, colored by chromium. It is one of the four precious gemstones and among the most valuable gems per carat in the world. Fine rubies from Myanmar (Burma) regularly sell for over one million dollars per carat.

• Refractive index: 1.762 to 1.770
• Birefringence: 0.008
• Pleochroism: Strong - purplish red to orangy red
• Crystal habit: Flat hexagonal tablets or barrel-shaped prisms
• Hardness: Mohs 9, second only to diamond

Ruby's trigonal structure is responsible for its strong pleochroism. When viewed along the c-axis, ruby appears more purplish red; perpendicular to the c-axis, it appears more orangy red. Gem cutters orient the stone to show the best color face-up.

Sapphire: Every Color of Corundum

Sapphire is any corundum that is not red. While blue sapphire is most famous, sapphire occurs in virtually every color including pink, yellow, orange, green, purple, and colorless (white sapphire).

• Blue sapphire: Colored by iron and titanium
• Pink sapphire: Colored by chromium (less than ruby threshold)
• Padparadscha: Rare pinkish orange variety, among the most valuable sapphires
• Color change sapphire: Appears blue in daylight, purple in incandescent light
• Star sapphire: Shows asterism (6-rayed star) due to rutile needle inclusions aligned with the trigonal structure

The 6-rayed star in star sapphires and star rubies is a direct result of the trigonal crystal structure. Rutile needles grow along three directions dictated by the 3-fold symmetry, creating the three sets of needles that produce the star effect.

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Quartz: The Most Common Trigonal Gem

Quartz (silicon dioxide) is the most abundant mineral on Earth and crystallizes in the trigonal system. It occurs in an enormous variety of gem forms, from transparent crystals to fine-grained aggregates.

Variety	Color	Notes
Amethyst	Purple	Most valuable quartz variety
Citrine	Yellow to orange	Often heat-treated amethyst
Rose Quartz	Pink	Rarely forms good crystals
Smoky Quartz	Brown to black	Color from natural radiation
Rock Crystal	Colorless	Pure quartz
Ametrine	Purple and yellow	Natural amethyst-citrine combination
Prasiolite	Green	Usually heat-treated amethyst

Quartz shows a unique property called optical activity - it rotates the plane of polarized light. Left-handed and right-handed quartz crystals rotate light in opposite directions, a property used in optical instruments.

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Tourmaline: The Rainbow Trigonal Gem

Tourmaline is a complex boron silicate that crystallizes in the trigonal system. It has the widest color range of any gem mineral and is famous for showing multiple colors within a single crystal.

• Rubellite: Red to pink tourmaline
• Indicolite: Blue tourmaline
• Paraiba: Neon blue-green, colored by copper - among the most valuable gems
• Watermelon tourmaline: Pink center with green rim, a single crystal
• Chrome tourmaline: Vivid green, colored by chromium

Tourmaline shows very strong pleochroism due to its trigonal structure. The color difference between the two pleochroic directions can be dramatic - a stone may appear dark green along the c-axis and light yellow-green perpendicular to it.

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Other Important Trigonal Gemstones

• Rhodochrosite: Pink to red manganese carbonate; forms rhombohedral crystals
• Calcite: Strong birefringence creates double images; used in optical instruments
• Hematite: Metallic gray-black iron oxide; used in jewelry and as a gemstone
• Phenakite: Rare colorless beryllium silicate; sometimes mistaken for diamond

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Identifying Trigonal Gemstones

• Refractometer: Two RI readings; uniaxial interference figure under polarized light
• Dichroscope: Strong pleochroism in corundum and tourmaline
• Polariscope: Anisotropic behavior confirms double refraction
• Spectroscope: Chromium absorption lines identify ruby and chrome tourmaline

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Frequently Asked Questions

What is the difference between ruby and red spinel?

Ruby is trigonal corundum (doubly refractive) while spinel is cubic (singly refractive). A polariscope or refractometer easily distinguishes them. Ruby also shows strong pleochroism; spinel does not.

Why does quartz show optical activity?

Quartz has a helical arrangement of silicon-oxygen tetrahedra in its trigonal structure. This spiral arrangement rotates polarized light as it passes through the crystal. Left-handed and right-handed quartz rotate light in opposite directions.

Why are Paraiba tourmalines so expensive?

Paraiba tourmalines contain copper as a chromophore, which produces an extraordinarily vivid neon blue-green color unlike any other gem. The original Paraiba mine in Brazil is nearly exhausted, making fine specimens extremely rare and valuable.

Can sapphire and ruby come from the same mine?

Yes. Ruby and sapphire are both corundum and form in the same geological environments. The Mogok Valley in Myanmar produces both world-class rubies and fine sapphires from the same marble deposits.

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Conclusion

The trigonal crystal system is arguably the most important in all of gemology. It gives us ruby and sapphire, the entire quartz family, and the endlessly varied tourmaline group. The 3-fold symmetry of trigonal crystals creates the star effects in star gems, drives the strong pleochroism in corundum and tourmaline, and produces the optical activity unique to quartz. Mastering the trigonal system is essential for anyone serious about understanding gemstones.