To inspect rough opal you need a good light source. Sunlight is good to detect colour that may be hidden under a surface coating, and is excellent for the basic sorting of material. However a desklamp with a higher wattage incandescent bulb is good to help determine facing direction, and to hold stones up to the rim so as to look through, and pick up inclusions.
Individual opals respond differently to different light sources - for example many lighter types of opal will fade under fluorescent light, whereas black opal will still show colour.
Each nobby has an opaque skin which hides the colour within - often this could be a layer of grey potch, which will need to be snipped off using tile snips, or sawn or ground off ( use the latter tools where expensive opal is involved).
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