ROUGH & CUT ETHIOPIAN OPALS
ETHIOPIAN GEM QUALITY OPALS
Global % of the rough worth processing (initial sawing) from a single stone to a batch.
% of gem quality opal detected in each rock or matrix with examination of outer layers, skin to skin outer layer being preferred.
Observation & assessment of rough pre-formed parcel.
I - Assessment whether the rough should be
- Cut and polished, or both including sawing.
- Weight (grams or carat) loss resulting form the cutting
- Poor, average to good yield.
Access to the color bar & check
- Access to the color bar & check
- Browns and chocolate are recorded showing high sensitiveness
- Thickness and depth of remaining pockets of host rock
- Visual appearance of skin (crust like)
- Sand penetration and contact with the color bar
- Removability of sand (hardness).
Analysis of fracture surface (different views)
- Links between cracks and porosity
- Breaks (brittle, conchoidal or uneven)
- Fire (diffracted color play) and crystal may show breakage
- Natural cracks and network (to differentiate from cracks inside the stone)
- Revealed crazes during or after initial sawing, and preforming process with assessment of size
- Tension cracks and their propagation when cutting the rough.
Porosity level according to soil heterogeneity
- Provenance & geological settings (Wollo province or Mezezo)
- Understanding of differences and characteristics of soil resulting in 2 to 10% water content variability and in extreme cases over 20%.
Crystallography - porous silica structure
- Arrangement of silica spheric particles observed with microscope
- Examination of void space
- Randomly or arranged orientation of the particles
- Chart of desirable and unwanted inclusions (cf.quartz crystals, plant fossils and others).
Color bar(s) - bands - layers are examined under a clear background as well as a dark background
Number observable and their morphology.
- Depth & main focus of color (fire)
- Thickness (bars are straight or uneven with distortion and ondulation)
- Brightness (transmission of color)
- First body tone assessment.
Edges of rough rocks are ground away, enabling the analysis of :
II - Observation & Assessment of cut
Water retention of cut opal & impact on stability
- Variability of porosity with gradual dehydration affecting the durability of the stone together with weight fluctuation
- White opals’ water retention : clouding visibility and final brigthness
- Incompleteness of running pattern (viewed face up mainly and back).
Hydrophane character (observable or not) - degree of completeness of drying process
Play of color (vivid or not)
Body tone distinct from play of color
- Near colorless crystal to milky white, black, brown, honey, fire
- Scarcity of body tone: bluish/white, darker greys, blacks
- Check of smoke concentration with magnification, indicative of treatment for black opals.
Brightness & scattering of light - fair to bright
Range of spectral colors examined on dry stones
- Chroma (purity/ intensity) & propagation
- Analysed from different angles with the naked eye (face up, edges, sides and back)
- Concentration of color with importance of voids or sections lacking color
- Dominant colors/variety up to three only
- More including conventional fire green, & red yellow for whites opals
- Broadness of each color flash
- Secondary colors with the final body tone assessment and clarity.
Contrast between body color & play of color - weak or strong
Pattern (face up)
- Homogeneity near surface (flash, pinpoint…)
- Broadness (flash)
- opaque to translucent & crystal.
Occurrence of surface & internal cracks - their importance is outlined, distinct from clean areas.