Rubies are the most expensive of the coloured stones, as high-quality stones over a carat are very rare. They are composed of the mineral corundum with traces of the element chromium, which gives it the signature red color. The color is the most important factor affecting the value of a ruby, to the point that a too-light ruby is considered a pink sapphire. Clarity is the next most important factor. Inclusions will bring down the value of a ruby but note that inclusion-free stones are almost non-existent.

As with sapphires and rubies, colour and clarity are the most important characteristics of emeralds, but they do vary from those stones in important ways. Sapphires and rubies are a 9 of 10 on the hardness scale, emeralds are more brittle. Eye- visible inclusions are accepted to be present even in high-quality emeralds, as the stones all have significant fractures. Stones that pass through the GIA receive ratings on the degree of clarity enhancement: minor, moderate or significant, though it’s not an actual clarity grade. Look for a true green to a bluish-green coloured emerald.

Sapphires come in many colours (and rubies are merely a red sapphire) but for the purposes of an engagement ring the most desirable sapphire is blue. Like rubies, colour and clarity, in that order, are the most important factors to consider. Note that deep blue sapphires will look slightly darker in the setting due to the light restriction. In addition, sapphires are heavier than diamonds, so a 1 carat sapphire will be smaller than a 1 carat diamond.