opal

Opal, a gemstone celebrated for its kaleidoscope of colours, derives its name from the Latin word "opalus," meaning "precious stone." This moniker aptly captures the enchanting allure of opals, which are composed of hydrated silicon dioxide.

Opals form through a unique geological process involving the percolation of silica-rich water into cavities in sedimentary rocks. Over time, as this water evaporates, it leaves behind tiny spheres that diffract light, creating the mesmerising play of colours known as "opalescence." This optical phenomenon is what makes opals so captivating.

Notably, opals are found in various regions, with Australia being a primary source, including the famous Lightning Ridge and Coober Pedy deposits. Ethiopia and Mexico also yield opals of exceptional beauty.

One fascinating fact about opals is that they can display a stunning range of colours, from milky white and fiery red to deep blue and vibrant green. This gemstone has long been associated with magic and mystery and continues to captivate gem enthusiasts with its unique charm.

Click on the terms in the table below to discover their meaning

Click on the terms in the table below to discover their meaning

Name Opal
Category Tectosilicate mineraloid
Chemical Formula SiO2 · nH2O
IMA Symbol Opl
Crystal System Amorphous
Crystal Habit Amorphous, massive, reniform
Twinning Absent
Cleavage None
Fracture Conchoidal
Tenacity Brittle
Hardness
(Moh's Scale)		 5.5 - 6.5
Specific Gravity 1.9 - 2.3
Diaphaneity Transparent to translucent
Colour Colourless, white, yellow, orange, red, blue, green, brown, black
Streak White
Lustre Vitreous, resinous or waxy
Optical Class Isotropic
Refractive index n = 1.400 - 1.460
Birefringence Absent; anomalous birefringence may be present due to stress-induced water loss
Dispersion Rd = 0.009 - 0.013
Pleochroism Absent
Clarity Type I
Notable Varieties Precious Opal, Crystal Opal, Fire Opal, Hydrophane, Hyalite, Prase Opal

References

Hudson Institute of Mineralogy. (2023). Opal. Retrieved from Mindat.org: https://www.mindat.org/min-3004.html

Mineralogy Database. (2012, September 5). Opal Mineral Data. Retrieved from Mineralogy Database: https://webmineral.com/data/Opal.shtml

About the Author

Daniel Gillespie is student of Web Development and the developer of Gemology 101. With a straightforward and informative approach, Daniel invites readers to explore the world of gemology, offering a valuable resource for both enthusiasts and those seeking knowledge about precious stones.