CLARITY AND TRANSPARENCY of coloured gemstones
– On the left side an emerald with inclusions and weak transparency. On the right a more pure and transparent crystal –
Clarity classification attributes a grade to the type and number of inclusions present in a gemstone. For inclusions we mean any feature trapped inside the crystal, from fractures to small crystals, fluids and gases.
Generally speaking, the absence of inclusions is more desirable and the gemstone becomes more valuable but in some cases it can help in the distinction between a natural or synthetic gemstone or be crucial in identifying its geographical origin.
Gemmological institutes have reached a very precise clarity classification for diamonds that is accepted by everyone. Diamond grading is carried out using a magnifying glass 10 X.
The situation is different for coloured gemstone because we are faced with different mineralogical species and finding an unequivocal approach is more difficult.
Gemstone clarity is evaluated considering the detrimental effect of the inclusions on the light transmission and therefore on the beauty and brilliance the gemstone.
Clarity GIA scale for coloured gemstone (Gemological Institute of America)
| Grade | Appearance to the unaided eye | |
| ECEye-clean | The stone appears clean to the unaided eye | |
| SISlightly Included | Tipo I | Minute inclusions difficult to see to the unaided eye |
| Tipo II | Minor inclusions somewhat easy to see with unaided eye | |
| Tipo III | Noticeable inclusions apparent to the unaided eye | |
| MIModerately
Included |
Tipo I | Minor inclusions somewhat easy to see with unaided eye |
| Tipo II | Noticeable inclusions apparent to the unaided eye | |
| Tipo III | Obvious inclusions very apparent to the unaided eye | |
| HIHeavily included | Inclusions are prominent with a negative effect on the appearance | |
Coloured gemstones are divided into three groups with different clarity standards
Type I: gemstones that are commonly clear, with no inclusions visible to the unaided eye. Includes: aquamarine, pink and yellow beryl, green tourmaline, blue zircon, tanzanite.
Type II: gemstones that typically show some inclusions which do not affect their beauty. Includes: alexandrite, peridot, garnet, iolite, spinel, quartz, zircon, corundum (excluding ruby) and all tourmalines (except green tourmalines).
Type III: gemstones that always have inclusions because of their genesis.
Includes: emerald, red beryl, ruby.
Transparency
The transparency of a gemstone is the degree to which the light passes through the gemmiferous material. Transparency is linked to clarity as inclusions within the stone may block or deflect the passage of light.
Transparency is described using the following terms:
transparent: objects viewed beyond the gemstone appear sharp.
semi-transparent: images appear slightly distorted and blurry.
translucent: the objects are hardly visible and images are distorted, light is still transmitted over the gemstone.
semi-translucent: only a small part of the light is transmitted over the gemstone.
opaque: light doesn’t pass through.
