B (Hue): Pure Blue in the GIA Coloured Stone Grading System
B (Hue): Pure Blue in the GIA Coloured Stone Grading System
The notation for pure blue on the GIA 31-hue circle, and its application in coloured gemstone colour description
In the GIA Colored Stone Grading System, B is the hue code designating pure blue as the dominant perceived hue of a coloured gemstone. Positioned at approximately 240° on the GIA 31-hue circle, it represents the anchor point of the blue region of the visible spectrum and serves as the reference against which blue gemstones with secondary hue components — such as green or violet — are measured and described. The notation is used in laboratory reports, grading certificates, and trade communication to bring systematic precision to what has historically been one of the most subjectively described colour regions in gemmology.
The GIA Hue Circle and the Position of B
The GIA Colored Stone Grading System divides the full colour wheel into 31 discrete hue positions, each assigned a letter code or paired-letter code. Pure hues — those without a perceptible secondary component — are designated by a single capital letter: R (red), O (orange), Y (yellow), G (green), B (blue), V (violet), and P (purple). Intermediate hues are expressed as paired codes in which the first letter denotes the secondary (modifier) hue and the second denotes the dominant hue; thus bB indicates a blue with a slight blue-violet or bluish cast that remains predominantly blue, while vB would indicate a violetish blue. Hues that fall precisely between two anchor points receive equal-weight codes such as GB (green-blue) or VB (violet-blue), written with both letters capitalised.
Within this framework, B occupies the position of theoretically pure blue — a hue with no discernible green, violet, or grey modifier. It is paired with separate descriptors for tone (on a scale from 0, colourless, to 10, black) and saturation (from 0, neutral grey, to 6, vivid) to produce a complete colour grade. A fully specified GIA colour description might therefore read: B 7/5, indicating pure blue at tone 7 and saturation 5.
Rarity of Pure B in Natural Gemstones
Achieving a grade of pure B in a natural gemstone is uncommon. The optical properties of most blue gem minerals introduce secondary hue components that shift the perceived colour away from theoretical purity. Blue sapphire (corundum), the most commercially significant blue gemstone, typically exhibits a slight violet or greenish modifier depending on its geographic origin and the lighting conditions under which it is viewed. Stones from Kashmir are frequently noted for a characteristic violetish blue — graded vB — while many Sri Lankan sapphires range from vB to bB. Aquamarine (beryl) commonly trends toward a greenish blue (gB) in its natural state, and heat treatment is routinely applied to reduce the green component and move the stone closer to a pure or slightly violetish blue. Tanzanite, a variety of zoisite, is strongly pleochroic and typically grades in the violet-blue to blue-violet range rather than pure B.
The practical consequence is that pure B grades, when they do occur, can represent a premium colour position for certain species — particularly sapphire — where the trade has historically associated a clean, unmodified blue with the finest stones from celebrated localities.
Application in the Trade
Laboratory reports issued by major gemmological laboratories, including GIA, may employ hue notation directly or translate it into descriptive language for client-facing documents. In either form, the underlying hue code provides a standardised reference that allows buyers, sellers, and appraisers to communicate colour with greater consistency than purely subjective terms such as "cornflower" or "royal" blue, which lack agreed definitions and vary between speakers and markets. The GIA system does not replace those evocative trade terms — which carry their own market weight — but it provides an objective framework against which they can be anchored.
For sapphires in particular, the interplay between hue code, tone, and saturation is central to valuation. A stone graded pure B at a medium-dark tone (approximately 6–7) and high saturation (5–6) corresponds broadly to what the trade describes as a fine to exceptional blue, and commands corresponding premiums in the wholesale and auction markets.