Cornflower or Centaurea:
What is a color?
There are three primary colors (red, blue, yellow) and three secondary colors, which are the result of mixing primary colors (purple, orange, green).
Nature often displays tertiary colors, such as red-orange, yellow-green, or blue-violet, which are a primary color mixed with a secondary color.
When a color is mixed with gray, white, or black, we need the (overlapping) definitions of saturation, hue and tints.
What is saturation?
Saturation expresses the attribute of perception of gray of the same lightness. All grays have zero saturation. Theoretically a 100% saturation means there is 0% gray in a color.
Nature does not come in 100% saturation, but the higher the saturation the more expensive the gem.
What is a hue?
Some hues, like red, pink and blue, are (today) considered being more valuable than others (e.g. yellow or purple). This has and continues to change with fashion, over time and between cultures.
What is a tint?
A color mixed with white is a tint. A tint is lighter and less saturated than a color without the addition of the white.
Generally speaking: the less tint, the better the hue, the higher the price.Exceptions are e.g. the padparadscha, the rhodolite or cornflower blue sapphire which are characterized by the combination of hue and tint.
How to describe a color?
You may describe a stone correctly as "blue (hue) mixed with 20% gray (saturation) plus a bit of yellow mixed with a lot of white (tint)".
Such descriptions are hard to imagine and not very attractive.
Therefore color professionals use more illustrative names such as "ivy green", "cornflower blue" or "salmon orange" in connection with attributes like "strong" or "vivid".
Because these color names refer to something common and natural they express color in a more accessible way.
For the evaluation of gemstones, saturation, hue and tint are summarized in two criteria:
Color grade and tone.
What is a color grade?
Color grade describes the strength of the main color compared to other colors visible in the stone.
A 100% color grade in blue for example would imply that there are no other colors (like purple or violet) visible in the stone.
If there was also no gray in the stone, we would have a 100% saturation with a 100% color grade, but such a stone has never been found.
Some varieties (e.g. the Padaparadscha), which are defined by a combination of main colors (e.g. pink and orange), will receive a high color grade from the purity of the combined main colors, meaning the absence of other colors e.g. brown.
Generally the rule applies: the purer the color the higher the grade. But grade is nothing without tone:
What is a color tone?
Any color grade has to be seen in combination with tone.
Color tone varies from "very light" to "very dark". It is the amount of black or white mixed into a color.
In the extremes a colored stone could be white (light 5) or black (dark 95) with just a hint of color.
Only grade and tone together describe color value sufficiently:
A stone might, for example, show a rather pure blue, free of green or violet, but it might be of a very light tone thus the blue is less strong. Or it might, in the opposite, be of such a dark tone, that it appears rather black than blue.
Gemstones with high color grades and light-medium to medium-dark tones fetch the highest prices.
Grade and tone are framed by color zoning, clarity, brilliancy and depth:
Untreated 2.7 carat sapphire:
Some stones show colors only in parts or layers. To describe the strength of this common but generally unwanted effect, we use four levels: