Turquoise and lapis lazuli are two ancient gemstones, widely admired across cultures for their captivating blue hues. This article aims to clarify their fundamental differences, distinguishing them by visual characteristics, geological origins, physical properties, and specific care requirements.
Visual Distinctions
Turquoise ranges from sky blue to greenish-blue, with its exact shade influenced by the presence of copper and iron. Copper usually produces bluer tones, while iron can lead to greener hues. A common feature is its matrix, remnants of the host rock appearing as black, brown, or gold veins, often creating a spiderweb pattern. Its luster can range from a waxy to a sub-vitreous (glass-like) appearance, depending on its porosity and texture.
Lapis lazuli is recognized for its intense, deep royal blue color. It often includes distinctive golden flecks of pyrite, which resemble tiny stars, and white streaks or patches of calcite. These inclusions are integral to its visual character. Its luster is generally described as dull, sub-vitreous, or greasy, differing from the waxy finish often seen in turquoise.
Geological Makeup and Formation
Turquoise is classified as a hydrous phosphate of copper and aluminum. This gemstone commonly forms in arid or semi-arid regions. It develops when acidic waters, containing copper, aluminum, and phosphorus, percolate through existing aluminous rocks, leading to the deposition of turquoise in cavities and fractures. Significant deposits are found in the United States and Iran.
Lapis lazuli is not a single mineral but a metamorphic rock composed mainly of lazurite. It also contains varying amounts of white calcite and golden pyrite. Lapis lazuli forms through contact metamorphism, where limestone or marble undergoes intense heat and pressure, often near igneous intrusions. The primary source for centuries has been Afghanistan, with other deposits in Chile and Russia.
Physical Characteristics and Durability
On the Mohs scale of mineral hardness, turquoise rates between 5 and 6, indicating a moderate level of scratch resistance. Its porosity can vary significantly; some turquoise is quite porous, which can affect its stability and susceptibility to color changes. Denser turquoise, with lower porosity, tends to be more durable and takes a better polish. It is sensitive to high heat, which can cause discoloration and surface damage, and it can react unfavorably with certain chemicals, cosmetics, and body oils.
Lapis lazuli has a Mohs hardness ranging from 5 to 5.5, comparable to turquoise. Its toughness is fair, and its density typically falls between 2.7 and 2.9 g/cm³. It is sensitive to harsh chemicals and extreme temperatures, which can lead to surface etching or fading of its vibrant blue color. Prolonged exposure to water can be problematic, potentially dissolving its calcite components or causing pyrite inclusions to rust.
Care and Authenticity
Due to its porosity and sensitivity, turquoise requires specific care to maintain its appearance. It is advisable to avoid exposing turquoise to chemicals such as lotions, perfumes, hairsprays, and cleaning products, as these can be absorbed and alter its color. Prolonged exposure to direct sunlight and high heat should also be avoided, as they can cause dehydration and color shifts. Gently wiping turquoise with a soft, damp cloth after wear helps remove surface oils and dirt, but soaking should be avoided.
Lapis lazuli also benefits from careful handling to preserve its deep blue hue. It should be kept away from harsh chemicals, abrasive materials, and extreme temperatures to prevent damage or fading. Cleaning lapis lazuli with a soft, damp cloth and mild soap is generally safe, but prolonged water exposure should be minimized. Storing both turquoise and lapis lazuli separately in soft pouches can prevent scratches from harder gemstones.
Many turquoise pieces on the market are treated to enhance their color, hardness, and durability, often through stabilization with resins. Dyeing is also a common treatment to improve color uniformity. Lapis lazuli can similarly undergo treatments like dyeing to intensify its blue and waxing or oiling to improve luster. Imitations of both gemstones exist, including dyed howlite or magnesite for turquoise, and various dyed materials for lapis lazuli. Understanding these treatments and imitations is important for assessing the authenticity and value of the stones.