Pearl and alexandrite are both precious gemstones that are highly valued for their beauty, rarity, and durability. However, while they share some common traits, there are also some key differences between pearls and alexandrite in terms of their composition, formation, colors, hardness, and value. Understanding the similarities and differences between these two gems can help appreciate what makes each one unique.
What are pearls and alexandrite?
Pearls are organic gemstones that form within certain mollusks, primarily oysters and mussels. They are composed of layers of calcium carbonate (primarily aragonite) that slowly build up around an irritant inside the mollusk’s mantle or shell. The iridescent sheen of pearls comes from light reflecting and refracting off the layers of aragonite platelets.
Alexandrite, on the other hand, is a rare variety of the mineral chrysoberyl that contains traces of chromium. Unlike pearls, alexandrite is an inorganic mineral that forms through natural geologic processes in the earth’s crust. While most chrysoberyl is yellowish-green, the chromium impurities in alexandrite cause it to exhibit an unusual color change – appearing green in daylight and red in incandescent light.
Composition and Structure
The key compositional difference between pearls and alexandrite is that pearls are organic gemstones composed of calcium carbonate, whereas alexandrite is an inorganic mineral composed of beryllium aluminium oxide (BeAl2O4) with chromium impurities.
Pearls have a layered structure consisting of thin crystalline aragonite platelets and conchiolin (a scleroprotein). The aragonite in pearls forms in concentric layers around the irritant over time. This layered structure is what produces the iridescent luster of pearls.
Alexandrite has a single crystalline structure with the chrysoberyl molecular lattice. The chromium ions substituted into the crystal structure are responsible for alexandrite’s distinct color change. Unlike the layered structure of pearls, alexandrite does not display iridescence.
Formation
Pearls and alexandrite also differ significantly in how they form. Pearls form through a biological process within mollusks when an irritant gets lodged in the mantle tissue. The mollusk secretes layers of aragonite and conchiolin to coat the irritant, creating the pearl over several years. Cultured pearls are formed by surgically implanting a bead into the mollusk to stimulate nacre secretion.
Alexandrite forms through extended geologic processes involving the crystallization of molten rock containing rare chromium-bearing minerals. Natural alexandrite is very scarce, found in only a few deposits worldwide. Synthetic alexandrite can also be grown in a lab through precise control of factors like temperature, pressure, and chemical environment.
Color
The colors of pearls and alexandrite differ dramatically. Pearls typically exhibit a soft iridescence and luster referred to as orient. Their background body color can range from white, pink, gray, yellow, blue, green, and black. High quality pearls display excellent luster and orient across their surfaces.
The signature feature of alexandrite is its remarkable color change from emerald green in daylight to ruby red under incandescent light. Natural stones also display a distinctive cat’s eye chatoyancy. The chromic ions in the crystal structure selectively absorb colors of light to produce this dramatic color change. Alexandrite’s colors are generally not iridescent.
Hardness
Pearls have a relatively soft hardness of 2.5-4.5 on the Mohs hardness scale. Their softness makes pearls prone to being easily scratched and damaged. Alexandrite is considerably harder, rating 8.5 on the Mohs hardness scale. This makes alexandrite a very durable gemstone, resistant to scratching and everyday wear. The hardness difference relates back to their underlying structure and composition.
| Gemstone | Hardness |
|---|---|
| Pearl | 2.5-4.5 |
| Alexandrite | 8.5 |
Value
Due to their differences in rarity, alexandrite is substantially more valuable per carat than pearls. Low quality pearls are abundant and inexpensive, while high quality natural pearls with excellent luster can be very expensive.
Natural alexandrite is one of the most scarce gemstones, even more so than diamonds. Top quality natural alexandrite can cost over $70,000 per carat. While cultured pearls are widely available, lab-grown alexandrite is also rare and costly to produce. Alexandrite’s color changing properties drive its mystique and value.
Jewelry Uses
Pearls and alexandrite are used in jewelry in different ways. Because of their softness, pearls are best suited to items like necklaces, earrings, and bracelets where they won’t be subject to as much abrasion. They need some care in wearing to avoid damage. Alexandrite is very durable, so can be used more flexibly in rings, bracelets, pendants, etc.
Pearls lend themselves to classic, elegant styles focused on their luminous quality. Alexandrite is well-suited for more ornate designer jewelry where its color change can be highlighted. Pearls complement a variety of outfits, while alexandrite offers a bold, dramatic pop of color.
Geographic Sources
The main sources of natural pearls are the oceans and freshwater rivers around the world. Saltwater pearling has historically focused in the Persian Gulf as well as waters around Australia and Indonesia. China dominates freshwater pearl production from rivers and lakes like the Yangtze. Japan also has a long history of pearl culturing and jewelry craftsmanship.
Natural alexandrite deposits are exceptionally limited, found mainly in Russia’s Ural Mountains, Brazil, Sri Lanka, and a few locations in East Africa. Russia has historically been the main producer of alexandrite, while Sri Lankan stones often display the most dramatic color change. Some other deposit locations yield chrysoberyl with less color change.
Imitations
Because natural pearls are expensive and rare, there are abundant low-quality pearl imitations on the market. Cheap simulated “pearls” can be made from materials like glass, plastic, or shell nuclei dipped in pearl essence. Imitation pearls lack the luster, density, and orient of real pearls. Careful examination is needed to identify fakes.
The rarity and value of alexandrite also make it a target for imitation. Synthetic alexandrite and color-change corundum may approximate alexandrite’s color change while lacking features like inclusions that identify natural alexandrite. Imitation alexandrite can also be made by combining green garnet and red spinel. Testing by gemologists using advanced equipment can detect these fakes.
Enhancements
Most pearls on the market have undergone some type of enhancement to improve their appearance, most commonly bleaching to whiten color and polishing to improve luster. Pearls may also be dyed to achieve more desirable colors. These enhancements are typically minor and standard industry practice, as long as disclosed properly.
Natural alexandrite is almost always sold unenhanced. Lab-created alexandrite may undergo some treatments, although this is rare since its synthetic origins are generally disclosed upfront. Unenhanced natural alexandrite commands the highest prices. Any enhancements in alexandrite should be disclosed, as it negatively affects value.
Conclusion
In summary, while pearls and alexandrite share similarities as rare and beautiful gemstones, important differences set them apart. Pearls have an organic animal origin and soft layered structure, while alexandrite is a hard crystalline mineral. Their colors, value, sources, and uses in jewelry also contrast. Pearls display subtle iridescence, while alexandrite exhibits a dramatic color change. Although both stones can be faked, imitation alexandrite is more common. While they make unique additions to any jewelry collection, pearls and alexandrite remain distinct in their intriguing natural properties.
