The "four Cs" were devised for diamond, where colour means absence of colour and clarity is measured against a near-flawless ideal. Applied to ruby, sapphire, emerald and the rest, the same four words mean different things and carry different weights. For a coloured stone, colour is most of the value, and the other three adjust it.
Colour
Colour is described in three parts.
Hue — the basic colour: red, orangey red, purplish red. Fine ruby is red with at most a slight purplish or orangey modifier; fine blue sapphire is blue to slightly violetish blue; fine emerald is green to slightly bluish green.
Tone — how light or dark, from very light to very dark. The most valued tones sit in the medium to medium-dark range; too light looks washed out, too dark looks black in poor light.
Saturation — how pure and intense the colour is, from greyish or brownish (low) to vivid (high). Saturation is the single strongest driver of value. A vivid medium-tone stone outranks a dull one of identical hue and tone many times over.
Colour is assessed face-up, in standardised daylight-equivalent lighting, against reference stones or a colour system. It is not a measurement; it is a trained comparison, and two graders can differ by a step. The laboratory can state hue, tone and saturation descriptively; any grade attached is an opinion.
Clarity
Coloured stones are graded against what is normal for the species, not against perfection. Emerald is expected to have inclusions (Type III in the common classification); aquamarine and blue topaz are expected to be nearly clean (Type I); ruby and sapphire sit between (Type II). A moderately included emerald may be "eye-clean for the type"; the same inclusions in an aquamarine would be heavy.
What matters is the effect on appearance: inclusions visible to the naked eye at normal viewing distance lower the grade; those visible only under magnification barely affect it. Inclusions that reach the surface or threaten durability are noted separately.
Clarity is where treatment lives: filled fractures in emerald and ruby are clarity enhancement, and the extent of treatment is graded alongside. Emerald oiling and resin filling.
Cut
Cut affects how the stone shows its colour and how much light it returns. For coloured stones the cutter's first job is to orient the rough so that the best colour faces up, then to proportion the stone so that colour is even across the face — no pale "window" through the centre, no dark "extinction" at the edges. Symmetry and polish quality are assessed as they are for diamond, but weight retention from the rough usually wins over ideal proportions, and slightly deep or shallow stones are normal.
Cutting styles include faceted (brilliant, step, mixed), cabochon (domed, for star and cat's-eye stones and for opaque material) and bead. The style is described; its quality is an opinion.
Carat weight
One carat is 0.2 gram. Price per carat rises with size for fine material — sharply for ruby and emerald, where large clean stones are scarce, and less steeply for species where large crystals are common. Weight is measured, not judged, and is the one C that is a hard number.
Note that density differs between species. A one-carat sapphire (SG 4.0) is smaller than a one-carat emerald (SG 2.7). Comparing stones by millimetre size and by carat weight are different things, and sellers sometimes choose whichever flatters the stone.
Why colour dominates
In diamond the four Cs are roughly co-equal. In coloured stones a fine-colour stone with moderate inclusions and an ordinary cut is worth far more than a flawless, superbly cut stone of dull colour. Grading systems for coloured stones weight colour accordingly, and a buyer choosing between stones should look at colour first, in good light, before anything else.
What a report gives you
A laboratory identification report describes colour in words, notes clarity characteristics and treatment, describes the cut, and measures weight and dimensions. It may or may not assign quality grades. Where it does, the grades are the examiner's opinion against a stated system and should be labelled as such — they are not the same kind of statement as the refractive index. What a laboratory report cannot tell you.
Frequently asked questions
Is there a universal grading scale for coloured stones?
No. Several systems exist and laboratories differ. A grade is meaningful only with its system named.
Is a lighter stone worse?
Lighter tone lowers value in most species, but the relationship is not linear and depends on the hue. A light pink sapphire can be more valuable than a dark, dull blue one.
Does an inclusion always reduce value?
Visible ones, yes. Some inclusions — silk that produces a star, the horsetail in demantoid garnet — are prized. And inclusions are what prove a stone natural.
Why does the same stone look different in the shop and at home?
Lighting. Shop lights are chosen to flatter. Judge colour in daylight near a window and under ordinary indoor light before deciding.
Should I prioritise weight or colour?
Colour. A smaller stone of fine colour is the better purchase for almost every purpose, and it holds its value better.