Yellow Opal

• opal is a mineral species; amorphous structure; chemical composition SiO2.nH2O (water content usually 3 to 10% by weight, but sometimes as high as 20%) 
• transparent to opaque
• yellow, milky yellow, cream yellow
• common opal, no play-of-color

Identification

• Optic Character: SR, ADR common due to strain 
• Refractive Index: 1.450 (+.020,-.080). Mexican opal may read as low as 1.37. but it is
• generally 1.42 to 1.43.
• Birefringence: none 
• Pleochroism: none 
• Absorption Spectra: green – 660 nm, 470 nm cut-off 
• Cause of Color: play-of-color – diffraction effects from tiny regularly stacked silica spheres: green (bodycolor) – nickel in clay impurities 
• Specific Gravity: 2.15 (+.08, -.90) 
• Polish Luster: vitreous to resinous 
• Fracture: conchoidal to uneven
  Luster: sub-vitreous to waxy (the latter is more typical of common opal) 
• Cleavage: none 
• Identifying Characteristics: play-of-color
• Crystal Habit: none, amorphous 
• Hardness: 5 to 6 1/2 
• Toughness: very poor to fair 

Enhancements

• Method: impregnation with oil, wax, or plastic
  

Effect: improves play-of-color, prevents or disguises cracking
Prevalence: common
Detection: oil and wax: hot point; plastic usually requires special laboratory equipment
(scanning electron microscope, infrared spectrometer)
Stability: fair to poor for oil or wax; excellent for plastic
Special Care Instructions: avoid solvents and re- polishing 

Major Sources: Australia, Brazil, Mexico. Others: Czechoslovakia, Honduras, Indonesia, Poland, Tanzania, US 

Stone information is reference from

Gem Reference Guide, Published by GIA. ISBN 0-87311-019-6

Gemstones of the World, Published by Sterling Publishing Co., Inc NY, ISBN 1-4027-4016-6

Encyclopedia of GEMSTONE, Published by ISHINSHA CO., LTD. ISBN 978-986-251-831-1