Tetragonal Crystal System Gemstones: Zircon and Apophyllite

The tetragonal crystal system may have fewer gem representatives than some other systems, but it produces some truly remarkable stones. Zircon, one of the oldest minerals on Earth, is the star of this system, offering extraordinary brilliance and dispersion that rivals diamond. Apophyllite, vesuvianite, and scapolite round out the tetragonal gem family.

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What Is the Tetragonal Crystal System?

The tetragonal system has three axes: two equal horizontal axes (a and b) and one vertical c-axis of different length, all intersecting at 90 degrees. Think of it as a cubic system that has been stretched or compressed along one axis, creating a 4-fold symmetry instead of the cubic system's full symmetry.

Key Characteristics of Tetragonal Gems

• Doubly refractive - uniaxial optical character
• 4-fold symmetry axis along the c-axis
• Crystal habits: square prisms, tetragonal bipyramids, combinations
• Pleochroism possible - two colors visible from different directions

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Zircon: The Ancient Tetragonal Gem

Zircon (zirconium silicate) is one of the most scientifically important minerals on Earth. The oldest known zircon crystals are over 4.4 billion years old, making them the oldest material ever found on our planet. As a gemstone, zircon is prized for its exceptional brilliance and fire.

Zircon's Remarkable Optical Properties

• Refractive index: 1.925 to 1.984 (high type); varies by metamict degree
• Birefringence: 0.059 - very high, causing strong facet doubling
• Dispersion: 0.038 - high, creating strong fire
• Luster: Adamantine to subadamantine

Zircon's high birefringence is one of its most distinctive features. When you look through the table of a faceted zircon, the back facet edges appear doubled - a clear sign of its tetragonal double refraction. This property alone can identify zircon in the lab.

Zircon Color Varieties

Color	Cause	Notes
Blue	Heat treatment	Most popular; from Cambodia and Myanmar
Colorless	Heat treatment of brown	Historically used as diamond substitute
Red-orange	Trace elements	Called hyacinth or jacinth historically
Yellow	Trace elements	Often called golden zircon
Green	Metamict structure	Low-type zircon; rare
Brown	Natural color	Most common natural color

Zircon vs Cubic Zirconia: A Critical Distinction

Zircon and cubic zirconia (CZ) are completely different materials that are frequently confused. Zircon is a natural mineral (zirconium silicate) that crystallizes in the tetragonal system. Cubic zirconia is a synthetic material (zirconium oxide) that crystallizes in the cubic system. They share part of a name but are otherwise unrelated.

Metamict Zircon

Some zircon contains uranium and thorium, which undergo radioactive decay over millions of years. The radiation gradually destroys the crystal structure, creating metamict zircon - a partially or fully amorphous material with lower refractive index and specific gravity than normal zircon. Heat treatment can partially restore the crystal structure.

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Apophyllite: The Collector's Tetragonal Gem

Apophyllite is a phyllosilicate mineral that crystallizes in the tetragonal system, forming beautiful square prismatic crystals with distinctive pearly luster on cleavage faces. While too soft for most jewelry (Mohs 4.5 to 5), apophyllite is highly prized by mineral collectors for its perfect crystal forms and exceptional clarity.

• Colors: Colorless, white, pale green, pink, yellow
• Crystal habit: Square prisms with pyramidal terminations
• Notable property: Strong pearly luster on basal cleavage; high transparency
• Sources: India (Pune region produces world-class specimens), Brazil, USA

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Other Tetragonal Gemstones

Vesuvianite (Idocrase)

Vesuvianite is a complex calcium silicate that crystallizes in the tetragonal system. The gem variety, called californite when it resembles jade, occurs in green, yellow, brown, and blue. A rare blue variety called cyprine is colored by copper.

Scapolite

Scapolite is a group of tetragonal silicate minerals that occur in yellow, orange, pink, purple, and colorless varieties. Some scapolite shows chatoyancy (cat's eye effect). It is an underappreciated gem with good brilliance and interesting colors.

Rutile

Rutile (titanium dioxide) crystallizes in the tetragonal system and has the highest birefringence of any natural mineral. While rarely used as a faceted gem due to its softness, rutile inclusions in quartz create the golden needles of rutilated quartz, and rutile needles in corundum create star rubies and star sapphires.

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Identifying Tetragonal Gemstones

• Refractometer: Two RI readings; zircon's high RI is distinctive
• Loupe or microscope: Facet doubling visible in high-birefringence zircon
• Polariscope: Anisotropic behavior confirms double refraction
• Specific gravity: Zircon has very high SG of 4.6 to 4.7, noticeably heavy for its size

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Frequently Asked Questions

Is zircon a good diamond substitute?

Colorless zircon has been used as a diamond substitute for centuries due to its high refractive index and dispersion. However, zircon is brittle and its facet edges chip easily with wear. It is better appreciated as a gem in its own right rather than as a diamond imitation.

Why does blue zircon look so brilliant?

Blue zircon's exceptional brilliance comes from its very high refractive index (around 1.93 to 1.98) and strong dispersion (0.038). Combined with its adamantine luster, this gives blue zircon a sparkle that rivals many more expensive gems.

Is zircon radioactive?

Some zircon contains trace amounts of uranium and thorium, which are mildly radioactive. However, gem-quality zircon used in jewelry contains such small amounts that it poses no health risk. The radioactivity is actually useful to scientists for uranium-lead dating of ancient rocks.

What is the rarest tetragonal gemstone?

Cyprine, the blue copper-bearing variety of vesuvianite, is among the rarest tetragonal gemstones. Fine gem-quality scapolite in deep purple is also quite rare and undervalued by the market.

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Conclusion

The tetragonal crystal system may be smaller than some others in terms of gem variety, but zircon alone makes it one of the most fascinating systems in gemology. As one of Earth's oldest minerals with extraordinary optical properties, zircon deserves far more recognition than it receives. Whether you are drawn to its ancient geological history, its diamond-like brilliance, or its wide color range, tetragonal zircon is a gem worth knowing deeply.