Color Grading of Blue Sapphire Gemstone

Color-Grading is broken into three quantifiable categories: intensity (saturation), hue (color), and tone (lightness/darkness). The GIA specifies thirty-one individual gemstone hues. Relating to sapphire, terms such as “blue,” “slightly greenish blue,” “very slightly greenish blue” are used to describe color tendencies. The color-grading nomenclature also specifies six levels of saturation ranging from “grayish” (neutral grey) to “moderately strong” to “vivid,” and nine levels of tone ranging from “very very light” to “very very dark.” A numerical value is assigned to each label for use in a gemstone color grading report.

The most desirable blue sapphire color is an intense, pure, and primary blue with a slight hint of violet and very little of the gray or green color components. For a blue sapphire to receive a perfect ’10’ quality rating it would have a “violetish/blue” hue, with a 6 or “medium dark” tone, and 6 or “vivid” color saturation level. Specific color grades of blue sapphire are commonly refers to as: Ceylon Blue, Cornflower Blue, Electric Blue, Kashmir Blue, Royal Blue, Sky Blue, Velvet Blue, and Violet Blue. High-quality Kashmir, velvet-blue and Cornflower-blue sapphires will maintain their color and intensity under a variety of lighting conditions from bright sunlight to dim artificial light.

Color zoning in most sapphire will exhibit moderate to strong color-zoning, caused by growth layers as the crystal is formed, however, sapphire from Burma may have very uniform color with little or no color zoning. Sapphire color-zoning may appear as concentric hexagonal rings (below, left), that run parallel to the prismatic outer facets of the rough crystal.

Identification

Color:                           Typically blue, but varies

Crystal habit:                As crystals, massive and granular

Twinning:                     Both growth twins and polysynthetic glide twinning on the rhombohedron

Cleavage:                     Poor

Fracture:                      Conchoidal, splintery

Mohs scale hardness:    9.0

Luster:                          Vitreous

Streak:                          Colorless

Diaphaneity:                 Transparent to nearly opaque

Specific gravity:             3.98~4.06

Optical properties:        Abbe number 72.2

Refractive index:           nω=1.768–1.772  : nε=1.760–1.763,

Birefringence:               0.008

Pleochroism:                 Strong

Melting point:               2,030–2,050 °C

Fusibility:                      Infusible

Solubility:                     Insoluble

Other characteristics:    Coefficient of thermal expansion (5.0–6.6)×10−6/K

Relative permittivity at 20 °C and ε = 8.9–11.1 (anisotropic)