Your Free Guide to Understanding Gemstones

What Are Gemstones and How Do They Form?

Gemstones are minerals, rocks, or organic materials that are valued for their beauty, rarity, and durability. They are used in jewelry, decorative items, and collections around the world. Understanding how gemstones form helps you appreciate their value and characteristics.

Most gemstones form deep within the Earth over millions of years through natural processes. The most common formation method involves heat and pressure in the Earth's crust. When rocks are subjected to intense temperature and pressure at great depths, the minerals within them can crystallize into gemstone-quality material. This process typically takes place between 3 to 200 kilometers below the Earth's surface.

Different gemstones form in different environments. For example, diamonds form in the mantle of the Earth under extreme pressure and heat, then are brought to the surface by volcanic eruptions. Rubies and sapphires form when the mineral corundum crystallizes in metamorphic rocks. Emeralds develop when beryllium-bearing fluids interact with rocks containing chromium and vanadium.

Some gemstones form from organic materials rather than minerals. Pearls develop inside oysters and mussels when an irritant is coated with layers of nacre, also called mother-of-pearl. Amber forms from fossilized tree resin that has hardened over millions of years. Coral grows from tiny marine animals and creates beautiful gemstone material in the ocean.

The location where a gemstone forms affects its characteristics. Gemstones from different geographic regions often have distinct colors and qualities. For instance, rubies from Burma are known for their deep red color, while rubies from Thailand tend to be darker. This geographic variation makes gemstones from different locations valued differently by collectors and jewelers.

Takeaway: Gemstones develop through natural processes over extended periods. Learning about formation methods helps you understand why certain gemstones are rarer and more valuable than others.

The Four Most Common Precious Gemstones

The term "precious gemstones" traditionally refers to four types: diamonds, rubies, sapphires, and emeralds. These four gemstones have been highly valued for centuries and command higher prices than other gemstones. Understanding their characteristics helps you recognize quality and authenticity.

Diamonds are the hardest natural gemstone, ranking 10 on the Mohs hardness scale. This exceptional hardness makes them resistant to scratching and ideal for everyday jewelry. Diamonds form from carbon under extreme pressure in the Earth's mantle. Most diamonds are colorless, but they can appear in various colors including yellow, brown, pink, and blue. The Kimberley Process, established in 2003, tracks diamond origin to prevent conflict diamonds from entering the market. According to the Gemological Institute of America, approximately 142 million carats of rough diamonds are produced annually worldwide.

Rubies are the red variety of the mineral corundum. They get their red color from the presence of chromium. On the Mohs hardness scale, rubies rank 9, making them very durable. High-quality rubies from Burma (Myanmar) are considered the most valuable, with prices sometimes exceeding those of diamonds per carat. Star rubies, which display a star-like pattern when viewed under certain light, are particularly prized by collectors.

Sapphires are also corundum, but they come in every color except red (red corundum is classified as ruby). Blue sapphires are the most well-known variety. Like rubies, sapphires rank 9 on the Mohs hardness scale. The most famous sapphires come from Kashmir, Burma, Sri Lanka, and Thailand. Sapphires can be heated to improve their color and clarity, which is an accepted practice in the gemstone industry.

Emeralds are the green variety of beryl. They rank 7.5 to 8 on the Mohs hardness scale, making them softer than diamonds, rubies, and sapphires. Because of this, emeralds require more careful handling. Most emeralds contain small fractures and inclusions, which gemologists consider normal. Emeralds are often treated with oils or resins to fill surface fractures and improve appearance. The finest emeralds come from Colombia, Zambia, and Brazil.

Takeaway: Understanding the properties of precious gemstones helps you make informed decisions about gemstone jewelry and collections. Each precious gemstone has distinct characteristics that affect its value and durability.

How Gemstone Quality Is Evaluated

Gemstone quality is evaluated using specific criteria that help determine value and authenticity. The most common evaluation system for colored gemstones considers four main factors: color, clarity, cut, and carat weight. For diamonds, this system is so widely used that it's called the "Four Cs."

Color is often the most important factor for colored gemstones. Gemologists evaluate color based on hue, tone, and saturation. Hue refers to the primary color (red, blue, green, etc.). Tone describes how light or dark the color is. Saturation measures the intensity or purity of the color. For example, a ruby's value increases significantly with a deep, pure red color. A sapphire with a vivid blue color commands higher prices than one with a pale blue appearance. Color grading often uses comparison stones and standardized lighting to ensure consistency.

Clarity refers to the presence or absence of inclusions and fractures. Inclusions are materials trapped inside the gemstone during formation. Some inclusions are visible to the naked eye, while others require magnification to see. A gemstone with fewer visible inclusions is typically considered higher quality and more valuable. However, some inclusions are normal and expected. For emeralds, the presence of small fractures is so common that perfectly clear emeralds are rare and extremely expensive.

Cut refers to how the gemstone is shaped and faceted. A well-cut gemstone reflects light effectively, creating sparkle and brilliance. The cut also affects how the color appears to the observer. Gemstones can be cut into various shapes including round, oval, cushion, emerald (step cut), and radiant. The quality of the cut depends on the skill of the lapidary (the person who cuts the gemstone). A superior cut can sometimes make a lower-quality gemstone more attractive than a poorly cut higher-quality stone.

Carat weight measures the mass of a gemstone. One carat equals 200 milligrams. Gemstone prices typically increase exponentially with carat weight, not linearly. This means that two 2-carat rubies are usually more expensive per carat than one 4-carat ruby of similar quality. This principle applies to most colored gemstones.

Takeaway: Familiarizing yourself with the four quality factors helps you understand why gemstones have different prices and value. These criteria are used by jewelers, gemologists, and appraisers worldwide.

Treatments and Enhancements in the Gemstone Industry

Many gemstones undergo treatments or enhancements after mining and before sale to consumers. Understanding common treatments helps you make informed purchasing decisions. Treatments range from commonly accepted practices to more controversial methods, and ethical jewelers disclose these treatments to customers.

Heat treatment is the most common and widely accepted enhancement. Heating gemstones at controlled temperatures can improve color and clarity. For sapphires and rubies, heat treatment is standard practice and rarely disclosed because it's considered routine. Heat treatment can lighten dark stones, deepen pale colors, or remove unwanted color tints. According to the American Gem Society, the majority of commercial rubies and sapphires have been heat treated.

Surface fracture filling involves injecting oils, resins, or glass-like substances into surface fractures to make them less visible. This treatment is common for emeralds because they frequently contain surface fractures. The treatment improves the gemstone's appearance but may not be permanent. Gemstones treated this way may require retreatment over time, especially if exposed to extreme temperature changes or harsh chemicals.

Irradiation exposes gemstones to radiation to change or enhance their color. This treatment is used for diamonds, topaz, and tourmalines. Irradiated gemstones are safe to wear, as the treatment produces stable color changes. However, irradiation treatment must be disclosed because it affects the gemstone's natural history and value considerations.