Natural Opal Rough · 39.8 ct · For Faceting · Yellow Opal

$78.26

Natural Yellow Opal rough weighing 39.80 ct and measuring approximately 29.9 × 18.9 × 17.8 mm, displaying an attractive orangey-yellow body colour with semi-translucent areas and natural internal variation. Untreated and suitable for lapidary evaluation, the specimen offers useful cutting potential for faceting or other specialist gemstone work.

1 in stock

NGB Reference: NGB-000821 SKU: RO-OPOP024YE-002-063 Category: Brand:
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This natural Yellow Opal is an attractive 39.80 carat rough gemstone displaying a warm orangey-yellow body colour with semi-translucent areas and subtle tonal variation throughout the material. Its substantial size and natural internal depth make it particularly interesting as rough material for faceting, cutting or specialist lapidary work.

Measuring approximately 29.9 × 18.9 × 17.8 mm, the specimen retains its natural irregular form, with visible fracture surfaces, uneven contours and internal structural variation characteristic of rough opal. The combination of warmer orange zones and lighter yellow areas gives the material a distinctive appearance and may offer different cutting possibilities depending on orientation and the cutter’s intended design.

Opal is a hydrated form of amorphous silica rather than a crystalline mineral. Its internal structure and water content contribute to its characteristic optical and physical behaviour. Unlike precious opal, this specimen is appreciated primarily for its saturated body colour, translucency and natural appearance rather than for play-of-colour.

The material is semi-translucent, allowing light to enter parts of the rough and reveal differences in density, colour and internal structure. These natural variations are important considerations when planning a finished stone and form part of the individual character of the specimen.

With a hardness of approximately 5.5 to 6.5 on the Mohs scale, opal requires more careful cutting, setting and handling than many harder gemstones. It can also be sensitive to impact, rapid temperature changes and dehydration, so controlled lapidary work and appropriate long-term care are recommended.

No indications of heating or other treatment are present. The stone is therefore presented as a natural, untreated Yellow Opal with its original colour, structure and rough characteristics preserved.

Overall Score: 85/100

NGB Grade: Very Good

Quality Evaluation

Natural Characteristics17.0/20
Inclusion Profile17.0/20
Faceting Potential18.0/20
Usable Yield / Shape Potential16.0/20
Overall Rough Quality17.0/20

Evaluation Notes

A strong natural Yellow Opal rough specimen with attractive orangey-yellow body colour, good semi-translucency and useful internal depth for lapidary evaluation. The material shows a substantial size, appealing colour concentration and enough visual variation to offer several potential cutting orientations. Natural fissures, uneven translucency and irregular fracture surfaces require careful planning and reduce overall cutting predictability, but they remain consistent with untreated opal rough. Overall, the specimen offers good faceting potential and solid visual appeal for its material type.

Species

Opal

Gemstone Variety

Opal

Gemstone Sub-variety

Yellow Opal

Color

Yellow

Treatment

Unheated, Untreated

Transparency

Semi-Translucent

Gemological Properties

Carat Weight39.8 ct
Dimensions29.9 × 18.9 × 17.8 mm
TransparencySemi-Translucent
ClarityNatural rough material
Refractive Index1.44–1.46
Specific Gravity1.98–2.25
Hardness5.5–6.5 Mohs
BirefringenceNone
Crystal SystemAmorphous
LusterVitreous to Waxy
Color DescriptionOrangey Yellow
Transparency NotesSemi-translucent, allowing partial light transmission through the rough, with natural variations in transparency and denser orangey-yellow areas visible throughout the material.
Natural CharacteristicsNatural irregular rough surfaces, conchoidal fracture textures, subtle internal veils, minor fissures, uneven translucency, and natural variations in orangey-yellow colour intensity. These features are consistent with untreated rough opal and contribute to the individual character of the specimen.

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