Orthorhombic Crystal System Gemstones: Topaz and Peridot
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The orthorhombic crystal system produces some of gemology's most beloved stones. Topaz, peridot, tanzanite, and alexandrite all crystallize in this system, each with distinctive optical properties shaped by their three unequal axes. From the imperial topaz of Brazil to the vivid green peridot of volcanic origin, orthorhombic gems offer remarkable variety and beauty.
What Is the Orthorhombic Crystal System?
The orthorhombic system has three unequal axes (a, b, and c) all intersecting at 90 degrees. Unlike the cubic system where all axes are equal, or the tetragonal system where two axes are equal, every axis in the orthorhombic system has a different length. This creates a rectangular box-shaped unit cell with three different dimensions.
Key Characteristics of Orthorhombic Gems
- Doubly refractive - biaxial optical character (two optic axes)
- Trichroism possible - three different colors visible from three directions
- Crystal habits: tabular, prismatic, striated prisms
- Cleavage often well-developed along specific planes
A key distinction: orthorhombic gems are biaxial, meaning they have two optic axes rather than one. This is different from trigonal and hexagonal gems, which are uniaxial. Biaxial gems show more complex optical behavior under polarized light.
Topaz: The Imperial Orthorhombic Gem
Topaz (aluminum fluorosilicate) is one of the hardest silicate minerals at Mohs 8. It crystallizes in the orthorhombic system as well-formed prismatic crystals, often with striated prism faces and a distinctive basal cleavage.
Topaz Color Varieties
| Variety | Color | Notes |
|---|---|---|
| Imperial Topaz | Orange with pink undertone | Most valuable; from Ouro Preto, Brazil |
| Pink Topaz | Pink to red | Rare natural color; often heat treated |
| Blue Topaz | Sky to London blue | Almost always irradiated and heat treated |
| White Topaz | Colorless | Common; used as diamond substitute |
| Mystic Topaz | Multicolor | Colorless topaz with thin film coating |
| Sherry Topaz | Yellow-brown | Natural color from Pakistan and Brazil |
Topaz's Critical Cleavage
Topaz has perfect basal cleavage in one direction, perpendicular to the c-axis. This is a direct result of its orthorhombic structure and the weak bonding between fluorine-bearing layers. Jewelers must be careful when setting topaz, as a sharp blow can cause it to split cleanly along this cleavage plane. Ultrasonic cleaners should never be used with topaz.
- Refractive index: 1.619 to 1.627
- Birefringence: 0.008 to 0.010
- Pleochroism: Moderate trichroism in colored varieties
- Specific gravity: 3.53, noticeably heavy for its size
Peridot: The Volcanic Orthorhombic Gem
Peridot is the gem variety of olivine (magnesium iron silicate), one of the most abundant minerals in Earth's mantle. Unlike most gems that form in the crust, peridot crystallizes deep in the mantle and reaches the surface through volcanic activity. It is also found in meteorites, making it one of the few gems with an extraterrestrial origin.
Peridot's Distinctive Properties
- Color: Always green, ranging from yellowish green to brownish green; colored by iron
- Refractive index: 1.654 to 1.690
- Birefringence: 0.036 - high enough to see facet doubling
- Hardness: Mohs 6.5 to 7
- Characteristic inclusions: Lily pad inclusions (discoid fractures around crystals)
Peridot Sources
- Zabargad (St. John's Island), Egypt: Ancient source mined for over 3,500 years
- San Carlos Apache Reservation, Arizona: Largest single source of peridot worldwide
- Pakistan (Kohistan): Produces large, fine-quality stones
- Myanmar: Fine stones from Pyaung-Gaung
- Meteorites: Pallasitic meteorites contain gem-quality peridot crystals
Tanzanite: The Trichroic Orthorhombic Gem
Tanzanite is the blue-purple variety of zoisite, found only in a small area near Mount Kilimanjaro in Tanzania. It is famous for its extraordinary trichroism - showing three distinct colors from three different crystal directions: blue, violet, and burgundy red.
- Refractive index: 1.691 to 1.700
- Pleochroism: Strong trichroism - blue, violet, and red-brown
- Treatment: Almost all tanzanite is heat treated to enhance blue-violet color
- Rarity: Found in only one location on Earth; estimated reserves may be exhausted within decades
Alexandrite: The Color-Change Orthorhombic Gem
Alexandrite is the color-change variety of chrysoberyl, crystallizing in the orthorhombic system. It appears green in daylight and red to purplish red in incandescent light - a dramatic color change caused by chromium's interaction with the orthorhombic crystal structure.
- Hardness: Mohs 8.5, very durable
- Color change: Green to red; finest stones show complete color change
- Sources: Russia (Ural Mountains, original source), Sri Lanka, Brazil, India
- Value: Fine alexandrite can exceed ruby and emerald in price per carat
Identifying Orthorhombic Gemstones
- Refractometer: Two RI readings; biaxial interference figure under polarized light
- Dichroscope: Trichroism in tanzanite and alexandrite is diagnostic
- Polariscope: Biaxial figure distinguishes from uniaxial trigonal and hexagonal gems
- Specific gravity: Topaz's high SG of 3.53 is distinctive
Frequently Asked Questions
Why is most blue topaz treated?
Natural blue topaz is extremely rare. The vivid blue colors seen in most commercial topaz are produced by irradiation followed by heat treatment of colorless topaz. Sky blue, Swiss blue, and London blue are all trade names for different intensities of treated blue topaz.
Can peridot be found in space?
Yes. Peridot (olivine) is found in pallasitic meteorites - stony-iron meteorites that contain gem-quality olivine crystals embedded in an iron-nickel matrix. These meteorites originate from the core-mantle boundary of differentiated asteroids.
Why is tanzanite only found in one place?
Tanzanite requires a very specific combination of geological conditions to form: the right chemical composition, precise temperature and pressure, and the tectonic activity of the East African Rift. This unique combination exists only in a small area of about 7 square kilometers near Merelani, Tanzania.
What makes alexandrite change color?
Chromium in alexandrite's orthorhombic structure absorbs light in a way that transmits both green and red wavelengths. In daylight (which is rich in blue-green light), the stone appears green. In incandescent light (which is rich in red wavelengths), it appears red. The human eye's sensitivity shift between lighting conditions enhances this effect.
Conclusion
The orthorhombic crystal system delivers some of gemology's most spectacular performers. Topaz's imperial orange, peridot's volcanic green, tanzanite's trichroic blue-violet, and alexandrite's magical color change all arise from the same fundamental geometry: three unequal axes at right angles. Understanding the orthorhombic system helps you appreciate not just the beauty of these gems, but the remarkable science that creates it.
