What is a Ruby?

What are the criteria that gemological institutes follow to determine if a sapphire is a ruby? Can there be pink rubies?

The process to determine a ruby is straightforward:

  1. Is it corundum? If no, not ruby, if yes then:
  2. Is it red? If yes, then ruby, if no then it is sapphire

There is no pink ruby, pink corundum is called pink sapphire. Any colour of sapphire that is not blue is generally known as fancy sapphire.

However, what is ‘red’ and when does light red become pink? Is reddish orange still red, etc is a matter of individual opinion. Both hue and tone have to be taken into account.

This is a colour hue chat:

GIA Hue Chart
Hue Chart used in GIA Color Gemstone Grading

As you can see, there is no ‘pink’. Pink is a lighter tone of red.

This is a tone chart:

Gemstone Tone Chart
Gemstone Grading Tone Chart

To make it confusing, tone is often referred to as saturation. While this chart says ‘saturation’, what it is really showing is tone:

Not a tone chart
Chart uses ‘saturation’ where it means ‘tone’

So when grading a stone as ruby rather than pink or fancy sapphire, it needs to be ‘red’ with medium or better tone. Subjectively, one grade difference is acceptable – what I might call ‘red’, you might call ‘slightly purplish red’, or what I might call ‘medium’ tone, you might call ‘medium light’, and that is ok.

But there is no absolute definition for ruby, some labs might class ruby as anything with red in it, others might say red only, or some might include orangish red and slight purplish red.

Just to be clear about ‘saturation’, the saturation scale as defined by the GIA goes from ‘brownish’ to ‘vivid’ for red gemstones. It should make no difference to the actual classification of whether a gem is ruby or not.

Confusing, right? Yes, at first. But once you get your ‘eye in’ for ruby, you can pretty much tell at a glance. A definition I like to use is that if corundum can return a red ‘flash’ (a property of ruby is it returns more red light than it receives), then it deserves to be called ruby.

This is a sample of ruby colour variation:

Ruby color variation
Ruby color variation

 

The best possible colour of ruby is called ‘pigeon blood’, but that is a whole other story.

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