Opal Body Tone
The body tone of an opal (also termed base tone or background
colour) refers to the relative darkness of the background which is contrasting
with the opal's
play-of-colour. It
is assessed face-up while ignoring the play-of-colour and may range from N1 (jetblack) to N9 (white) as per AGIA body tone
chart:
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Black Opals show a play-of-colour within or on a
black body tone compliant with N1 to N4, when viewed face-up. |
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Dark Opals show a play-of-colour within or on a
dark body tone compliant with N5 to N6, when viewed face-up (these are
also known as Semi-black Opals). |
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Light Opals show a play-of-colour within or on a
light body tone compliant with N7 to N9, when viewed face-up (the N9
category is also referred to as White Opal). |
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AGIA
Body tone chart (approximation) |
Note: Body tone is usually not applicable to almost
colourless, transparent
Crystal Opals (sometimes also termed Water Opals) since
it would be completely affected by the colour of the surface on which it is
examined (e.g. in front of a white or black background).
However dark coloured, transparent Crystal Opals may be graduated in the first
approximation lying on a
white sheet of paper which demonstrates the worst case.
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