P (hue) — GIA Code for Pure Purple in Coloured-Stone Grading
P (hue) — GIA Code for Pure Purple in Coloured-Stone Grading
The unmodified-purple position on the GIA hue circle, distinct from slpR, vP, and rP
In GIA's systematic colour-grading vocabulary for coloured stones, the single letter P is the hue code for pure purple — purple as the dominant hue with no detectable secondary modifying hue. P sits between rP (reddish purple) and vP (violetish purple) on the GIA hue circle, with bP (bluish purple) on the cooler side and slpR (slightly purplish red) on the warmer side. Used on laboratory reports and in colour-description language, the P code is a precise statement that the stone reads as straight purple to a trained grader under the laboratory's standard viewing conditions.
The GIA hue circle
GIA's hue notation works around a thirty-one-position circle, with each principal hue (R, RP, P, vP, bP, B, gB, BG, G, yG, GY, Y, oY, O, rO, R) given a single- or two-letter code and the spaces between them filled by modifier-prefixed codes (sl-, st-, vR or vP for very, and so on). The system is designed so that a description like strongly purplish red (stpR), red (R), or slightly purplish red (slpR) carries unambiguous meaning across grading reports and across graders. P denotes the purple position itself — the centre of the purple sector, between the reddish-purple and violetish-purple modifiers.
Hue is one axis of the colour grade. The other axes are tone (lightness to darkness, on a 0-to-10 scale where 2 is very light and 8 is very dark) and saturation (intensity of colour, on a 1-to-6 scale from greyish through to vivid). A complete colour grade combines all three: a fine amethyst might be graded P 5/4 (purple, medium tone, moderately strong saturation), and a different stone of the same overall colour might be graded vP 5/4 if the violet character were just detectable above the underlying purple. The P code by itself answers only the hue question.
Stones graded as P
Pure purple is the typical hue for the finest amethyst, particularly Siberian-type material with the Russian-influenced reference standard, although most amethyst grades closer to bP or slpR depending on the stone. Purple sapphire ranges from P through bP and vP, with the most desirable stones often falling at P with strong saturation. Tanzanite is normally graded as bV or vB rather than P, but some Tanzanian zoisite reads as P or vP under appropriate viewing conditions. Iolite, fluorite, kunzite (toward the violet rather than the pink end), and other violet-purple species can be graded P when the underlying hue is unmodified.
Chemical and structural causes of pure purple vary by species. In amethyst it is iron-related colour centres modified by natural irradiation; in purple sapphire it is a combination of titanium-iron charge-transfer (the blue component) and chromium (the red component); in tanzanite it is the polychromatism of vanadium-bearing zoisite. The grading code does not record the cause; it records the perceived hue.
Use in trade and laboratory reports
Laboratory reports issued by GIA and by laboratories using the GIA colour vocabulary will state the hue code, the tone number, and the saturation number on the colour-grading line. Trade buyers and dealers read these codes as a precise summary of the stone's colour position; for purple stones the difference between P, slpR, vP, and bP is meaningful because each modifier shifts the desirability of the stone within its species. Pure-purple amethyst and sapphire are generally more desirable than purplish-red or strongly violetish stones in most market segments, with the preference reversing in some markets and species (Russian taste favours slpR amethyst, for example).
The trade also uses descriptive equivalents — straight purple, true purple, neutral purple — that map approximately onto P. The laboratory code is the more precise term and is the one that should be used in formal documentation and correspondence.