The world of blue gemstones offers a spectrum of colors, ranging from the deep, velvety midnight of a rare sapphire to the pale, tranquil sky of aquamarine. These captivating hues are found across the mineral kingdom, presenting themselves in stones that vary dramatically in prestige, durability, and cost. Understanding the distinctions between these minerals, from their composition to their accessibility, provides valuable insight into what makes each shade of blue unique in the world of jewelry.
Iconic and Highly Valued Blue Gems
The blue sapphire stands as the pinnacle of highly valued blue gemstones, a variety of the mineral corundum second only to diamond in hardness. The most sought-after colors are the velvety blue to violetish-blue tones with vivid saturation, often with a medium-dark tone. Historically, the Kashmir region has produced the most prized specimens due to their intense, almost luminous blue color, though sources in Sri Lanka, Madagascar, and Thailand are also significant.
High-quality tanzanite, a member of the zoisite mineral group, is another highly prized blue gem. It is exclusively sourced from a small area near Mount Kilimanjaro in Tanzania, which contributes to its rarity and high value. This gemstone is known for its trichroism, meaning it can display three different colors—blue, violet, and burgundy—when viewed from different angles. Most tanzanite is naturally a brownish color and must be heat-treated to achieve its desirable blue-violet hue, though the extremely rare, naturally blue stones command the highest prices.
Rare blue spinel offers an intense color that can rival sapphire, but it is a distinct mineral species. The most vivid blues are caused by trace amounts of cobalt, leading to the term “cobalt spinel” in the trade. These deep blue spinels are highly valued, with prices for fine specimens reaching thousands of dollars per carat, especially those from sources like Myanmar and Sri Lanka. Spinel is known for its single refractivity, providing a clean, bright appearance, and its Mohs hardness of 8 makes it a durable choice for jewelry.
Common and Accessible Blue Gems
Blue Topaz and Aquamarine
Blue topaz is the most common and affordable of the transparent blue gems, with its natural color often being pale or colorless. Most bright blue topaz is the result of a permanent treatment process involving irradiation and subsequent heating. This produces the popular Swiss Blue (a vibrant, bright blue) and London Blue (a deeper, inky blue) varieties. Its Mohs hardness of 8 ensures good durability for everyday wear.
Aquamarine, a light blue variety of the mineral beryl, is another popular choice, often displaying a delicate, pale, greenish-blue shade reminiscent of seawater. Found in deposits across the world, including Brazil, Nigeria, and Pakistan, aquamarine is commonly cut into transparent, faceted stones like the emerald cut to showcase its clarity and light color. The presence of trace iron is responsible for its blue hue, and heating is often used to remove any subtle green undertones, enhancing its pure blue appearance.
Lapis Lazuli and Turquoise
Lapis Lazuli is a rock aggregate composed primarily of the blue mineral lazurite, often speckled with golden flecks of pyrite and streaks of white calcite. The finest material, a deep, uniform ultramarine color, has been mined for millennia in Afghanistan’s Sar-e-Sang deposits. It is typically cut into smooth, polished cabochons or beads.
Turquoise, a hydrous phosphate of copper and aluminum, is known for its distinct color, which ranges from sky blue to greenish-blue, caused by the presence of copper. Found in arid regions like the American Southwest and Iran, turquoise is nearly always cut as a cabochon to highlight its color and unique matrix, which is the remnant of the host rock.
Understanding the Blue Color
The appearance of blue in a gemstone is caused by tiny amounts of foreign elements, known as chromophores, or by imperfections in the crystal lattice. In blue sapphire, the color is generated by a process called intervalence charge transfer, involving iron and titanium trace elements. Specifically, an electron is shared between adjacent ferrous iron (Fe2+) and titanium (Ti4+) ions, absorbing yellow and red light and allowing blue light to pass through.
For other blue gems, the color is directly linked to a specific element’s absorption of light. The vivid blue of cobalt spinel, for example, is attributed to the presence of cobalt ions acting as the chromophore. The blue in tanzanite is caused by vanadium trace elements within the zoisite structure, which, when heated, change their oxidation state to produce the desired blue-violet shade.
Practical Considerations for Buyers
Durability and Wear
Buyers should prioritize durability, which is measured using the Mohs scale of hardness. Gemstones with a hardness of 7 or higher, such as sapphire (9) and topaz (8), are suitable for daily wear jewelry like rings, as they resist scratching from common dust and grit. Softer stones, including tanzanite (6-6.5) and lapis lazuli (5-5.5), require protective settings and are best suited for pendants or earrings to prevent damage.
Treatments and Simulants
Consumers must be aware that enhancement treatments are commonplace and accepted across the industry. Most blue topaz is irradiated and heated for color, while blue sapphires and tanzanite are routinely heat-treated to improve color and clarity. Sellers should disclose these treatments, as they can affect the stone’s value and sometimes its care requirements. Finally, caution is necessary to avoid simulants, such as inexpensive blue glass or synthetic spinel. Synthetic blue spinel can sometimes be detected by a gemologist through its slightly different refractive index or the presence of curved growth lines and gas bubbles when viewed under magnification.