Serendibite is classified as a borosilicate mineral containing calcium, magnesium, and aluminum, with the complex formula \(\text{Ca}_2(\text{Mg}, \text{Al})_6(\text{Si}, \text{Al}, \text{B})_6\text{O}_{20}\). The presence of boron results from the unique geological environment—skarns affected by boron metasomatism—where it forms. Because this gem shares visual similarities with other minerals, accurate identification requires a methodical approach that goes beyond simple observation.
Visual and Physical Characteristics
Identification begins with properties observable with the naked eye or basic testing tools. Serendibite displays a color palette ranging from grayish blue-green to deep blue, but it is often found in dark green, brown, or nearly black hues, particularly in material sourced from Myanmar. When polished, the stone exhibits a vitreous luster, giving it a bright, glassy appearance.
Serendibite belongs to the triclinic crystal system, typically appearing granular or tabular, though sometimes found as small, fractured crystals. On the Mohs scale of hardness, Serendibite registers between 6.5 and 7. This means the stone is moderately durable and capable of scratching glass. Identification is aided by observing the absence of cleavage planes, which would indicate a tendency to split along specific directions.
Definitive Gemological Testing
Conclusive identification requires specialized gemological equipment to measure the stone’s optical and physical constants. A refractometer is used to determine the Refractive Index (RI). Serendibite’s RI typically falls within the narrow range of 1.701 to 1.706, a highly specific reading for this borosilicate composition. Measuring the RI is a primary step in separating Serendibite from visually similar, but optically different, minerals.
Measuring the Specific Gravity (SG) provides another definitive physical constant, often performed using hydrostatic weighing. Serendibite exhibits an SG range of 3.42 to 3.52, reflecting its density. This density measurement is valuable because the SG is directly tied to the mineral’s chemical formula and crystal structure. These precise numerical values confirm the identity when visual characteristics are inconclusive.
Serendibite is optically biaxial and displays strong pleochroism. Using a dichroscope, one can observe three distinct pleochroic colors, confirming its triclinic crystal structure. The colors commonly observed are yellowish-green, greenish-blue, and dark blue or violet-blue, a unique combination that strongly indicates Serendibite.
Distinguishing Serendibite from Look-Alikes
Serendibite’s dark coloration means it is frequently confused with several other minerals, requiring careful comparison of gemological properties. Black Spinel is singly refractive (isotropic), meaning it is non-pleochroic and has a single RI value of around 1.718. This contrasts with Serendibite’s doubly refractive, trichroic nature and range of RI values. Spinel is also significantly harder (Mohs 8) and possesses a slightly higher SG of approximately 3.60.
Black Tourmaline (Schorl), another dark mineral, presents a much lower RI range, typically between 1.625 and 1.672, which is well below Serendibite’s minimum value. Tourmaline is also uniaxial, showing strong dichroism (two colors) rather than Serendibite’s trichroism, and has a lower SG, ranging from 3.00 to 3.26. The difference in density makes the hydrostatic weighing test an effective separator.
Sapphirine
The distinction from Sapphirine is more subtle, as their SG and RI values overlap considerably (SG 3.40–3.58; RI 1.701–1.734). While Sapphirine is also biaxial and strongly trichroic, it is slightly harder, registering 7.5 on the Mohs scale. Its pleochroic colors, such as sky-blue, lavender-blue, and blue, do not always match Serendibite’s characteristic yellowish-green component.
Zoisite
Zoisite varieties, such as tanzanite, are also biaxial and trichroic. However, their RI range is slightly lower, typically falling between 1.691 and 1.700, and their SG is often lower, between 3.10 and 3.45. The specific pleochroic colors and precise RI measurements confirm Serendibite’s unique identity from these closest gemological analogues.