Spinel
Chemical formula: MgAl₂O₄
Spinel is an oxide mineral, valued in jewelry and collecting for its variety of colors and high hardness.
Description
## Characteristics Spinel is a mineral belonging to the oxide group, whose name comes from the Latin word "spina", meaning thorn, referring to its sharp, octahedral crystals. It occurs as well-formed, often twinned crystals, most commonly in the form of octahedra, less frequently rhombic dodecahedra or combinations thereof. Spinel specimens are characterized by a vitreous luster and can exhibit a wide range of colors, from colorless, through pink, red, purple, blue, green, brown, to black. ## Physical Properties Spinel is characterized by a Mohs hardness of 7.5-8, making it a durable and scratch-resistant mineral. It has a vitreous luster. Its density is approximately 3.6 g/cm³. It is a transparent to translucent mineral. The streak of spinel is grayish-white. ## Colors and Varieties Spinel occurs in many colors, which is a result of the presence of various trace elements in its structure. Red spinels owe their color to chromium, blue ones to iron and cobalt, and purple and pink ones to manganese. Known varieties include ruby spinel (red), sapphire spinel (blue), and pleonaste (black, rich in iron). There are also spinels that change color depending on the lighting. ## History and Name The name "spinel" comes from the Latin word "spina", meaning thorn, referring to the sharp shapes of its crystals. For centuries, red spinels were often mistaken for rubies, and many famous "rubies" in royal collections turned out to be spinels, such as the "Black Prince's Ruby" in the British Imperial State Crown. Spinel was known and valued in antiquity, especially in Asia, where it was mined from alluvial deposits. ## Applications Spinel is a prized gemstone used in jewelry. Its high hardness and variety of colors make it an attractive material for faceting. Black spinels are sometimes used as ornamental stones. Synthetic spinel finds application in the optical and electronic industries due to its physical properties.
Diagnostic features
## Identification Spinel can be identified by its characteristic, often well-formed octahedral crystals, vitreous luster, and high hardness. The variety of colors is also a diagnostic feature. It is worth noting the absence of cleavage. ## Distinguishing from Similar Minerals Red spinel is sometimes confused with ruby, but ruby has a Mohs hardness of 9 and often exhibits dichroism, which spinel does not. Blue spinel can be confused with sapphire, which is also harder (9 on the Mohs scale) and has a different crystal structure. Garnets, although also occurring in octahedral form, have lower hardness and different density. ## Crystal Forms Spinel most commonly occurs as single or twinned octahedral crystals. Rhombic dodecahedra or their combinations are less common. Crystals can be euhedral, with well-developed faces, or anhedral, in the form of granular aggregates.
Geological environment
## Genesis Spinel forms in metamorphic rocks, such as marbles and gneisses, as well as in some igneous rocks, especially basic and ultrabasic ones. It often occurs in alluvial secondary deposits, where it accumulates due to its resistance to weathering and high density. ## Mineral Associations Spinel often co-occurs with minerals such as corundum (ruby, sapphire), garnet, forsterite, chondrodite, calcite, dolomite, and phlogopite. In alluvial deposits, it is often accompanied by zircon and rutile. ## Locations Significant spinel deposits are found in Myanmar (Burma), Sri Lanka, Vietnam, Tanzania, Madagascar, Afghanistan, and Brazil. Historically important deposits were also in Tajikistan (Badakhshan).
Rarity
Very Common
Collector aspects
## Quality Criteria The most prized spinel specimens are those with intense, pure colors, especially red, pink, and blue. Transparency, absence of eye-visible inclusions, and a well-formed crystal shape are also important. Large, faceted stones of excellent color and clarity command the highest prices. ## Market Prices Spinel prices vary widely and depend on color, clarity, size, and origin. Red and pink spinels from Myanmar and blue ones from Vietnam and Tanzania are among the most expensive. Small, clean stones can cost from tens to hundreds of dollars per carat, while large, high-quality specimens can fetch thousands of dollars per carat. ## Popular Localities The most sought-after spinel specimens come from Mogok in Myanmar, where famous red and pink spinels are mined. Other prized localities include Luc Yen in Vietnam (blue and purple spinels), Mahenge in Tanzania (pink and red), and Ratnapura in Sri Lanka.
Care and storage
## Cleaning Spinel can be cleaned with lukewarm soapy water, using a soft brush to remove dirt. After washing, rinse the mineral thoroughly with clean water and dry with a soft cloth. ## What to Avoid Avoid contact of spinel with strong acids and bases, which can damage its surface. Despite its high hardness, spinel can be damaged by sudden impacts. Prolonged exposure to intense sunlight can, in some cases, affect the intensity of its color, especially in light-sensitive varieties. ## Storage Spinel should preferably be stored separately from other, harder minerals to avoid scratches. It is ideal to store it in a soft pouch or a separate compartment in a jewelry box. Avoid sudden changes in temperature and humidity.