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

- Sawed
- 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.
Edges of rough rocks are ground away, enabling the analysis of :


- Depth & main focus of color (fire)
- Thickness (bars are straight or uneven with distortion and ondulation)
- Brightness (transmission of color)
- First body tone assessment.



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…)
- Richness
- Broadness (flash)
- Scarcity
- opaque to translucent & crystal.

Occurrence of surface & internal cracks - their importance is outlined, distinct from clean areas.