Diaspore

Chemical formula: AlO(OH)

Aluminum oxide hydroxide, valued for its transparent crystals and the rare, color-changing gem variety known as Csarite (Zultanite).

## Characteristics Diaspore is an aluminum oxide hydroxide, most commonly forming flattened, tabular, or elongated, bladed crystals. It also occurs in lamellar, scaly aggregates, and less frequently in granular or massive forms. The mineral is characterized by a vitreous luster, which transitions to pearly on perfect cleavage surfaces. It is brittle despite its relatively high hardness. ## Physical Properties Its Mohs hardness ranges from 6.5 to 7, and its density varies from 3.3 to 3.5 g/cm³. It exhibits perfect cleavage in one direction, making it susceptible to impact. It is a transparent to translucent mineral. ## Colors and Varieties Diaspore comes in various colors: it can be colorless, white, grayish, yellowish, greenish, lilac, or pink. The most prized variety is the gem material from Turkey, known by the trade names **Csarite** or **Zultanite**. This variety exhibits strong pleochroism and a color-change effect—it is green in daylight (similar to kiwi) and becomes pinkish-red or raspberry in incandescent (artificial) light. ## History and Name The name diaspore comes from the Greek word *diaspeirein*, meaning "to scatter," referring to its property of cracking and breaking apart with a crackling sound when heated in a flame. The mineral was first described in 1801 by the French mineralogist René Just Haüy. ## Uses Diaspore is primarily a mineral of interest to collectors. Its noble, color-changing variety is a highly valued and rare gemstone. It also constitutes one of the main components of some bauxite deposits, making it an aluminum ore.

Properties

Mohs hardness
6.5-7
Luster
Vitreous, pearly on cleavage faces
Streak
White
Density
3.3-3.5
Cleavage
Perfect on {010}, good on {110}
Fracture
Conchoidal
Transparency
Transparent to translucent
Crystal system
Orthorhombic

Diagnostic features

## Identification Characteristic features of diaspore include its high hardness (6.5-7), perfect cleavage in one direction, and bladed or tabular crystal habit. The luster is vitreous, but distinctly pearly on cleavage surfaces. Gem varieties can be easily identified by strong pleochroism and characteristic color change under different lighting conditions. ## Distinguishing from Similar Minerals Diaspore is sometimes confused with sillimanite, kyanite, or andalusite, but differs from them in crystal habit, cleavage angles, and optical properties. Brucite is significantly softer (hardness 2.5). Transparent varieties may resemble some topazes or beryls, but differ from them in density and optical characteristics. ## Crystal Forms Diaspore crystals are typically elongated along one axis and flattened, forming bladed, tabular, or acicular habits. They often occur as radial or lamellar aggregates. Less commonly, it is found in granular aggregates or massive forms.

Geological environment

## Genesis Diaspore is a mineral formed by metamorphic processes in aluminum-rich rocks, such as crystalline schists, marbles, or emery deposits. It also forms in hydrothermal alteration zones and as a weathering product of aluminous minerals in laterites and bauxites, where it is one of the main components of aluminum ore. ## Mineral Associations It often co-occurs with minerals such as corundum, gibbsite, boehmite, spinel, magnetite, chlorites, calcite, and dolomite. ## Localities The world's most famous deposit of gem-quality, color-changing diaspore is located in the Ilbir Mountains in the Milas district (Muğla province) in Turkey. Other significant historical localities include Chester, Massachusetts (USA), where it occurred in emery deposits, the vicinity of Banská Štiavnica in Slovakia, and the Urals in Russia.

Rarity

Not very common

For collectors

## Quality Criteria For gem varieties (Csarite/Zultanite), the most important criteria are the intensity and distinctness of the color change, clarity (lack of inclusions), and stone size. Specimens weighing over 5 carats are considered rare and very valuable. For mineralogical specimens, well-formed, transparent crystals with vivid color, set on a rock matrix, as well as visually attractive aggregates, are sought after. ## Popular Localities Specimens from Turkey are by far the most prized by collectors and jewelers. This is the only place in the world where gem-quality, color-changing material is mined. Specimens from classic localities, such as Chester in the USA, primarily have historical and scientific value.

Care and storage

## Cleaning Diaspore specimens are best cleaned with lukewarm water and a mild soap, using a soft brush. After washing, rinse thoroughly with clean water and allow to air dry. ## What to Avoid Due to its perfect cleavage, the mineral is sensitive to impact and pressure. Avoid ultrasonic and steam cleaners, which can cause fractures along cleavage planes. It is resistant to most chemicals, but contact with strong acids and bases should be avoided. ## Storage Diaspore specimens, especially faceted stones, should be stored separately in soft pouches or padded boxes. This protects them from scratching by harder minerals and from accidental impacts that could cause cleavage.

External references

Sources

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