Name/Title
Opal in MatrixEntry/Object ID
2013.1.18Description
Chemical Composition: SiO2
Crystal System: Triclinic System
Description: Common Name: Opal
Chemistry: SiO2·nH2O
Group: Hydrated Silica
Location: Quilpie Mine, west of Brisbane, Australia
Description: This a small boulder sized rock with the opal running through the middle of one face. (unpolished)
Physical Description:
•Color is white, colorless, pale yellow, pale red, gray or black when impurities are common. Diffraction can cause flashes of any color of the rainbow (play of light).
•Luster is vitreous to pearly.
•Transparency: Specimens are transparent to translucent (opalescent).
•Crystal System: Does not apply because opal is amorphous.
•Habits include massive, cavity-fillings such as in fractures and geodes, nodular, reniform or as a replacement of other minerals and wood.
•Cleavage is absent.
•Fracture is conchoidal.
•Hardness is 5.5 - 6
•Specific Gravity is approximately 2 - 2.5 (light)
•Streak is white.
•Other Characteristics: Most specimens will fluoresce white or pale green, some phosphoresce and all specimens can be very sensitive to impacts and low temperatures.
•Associated rocks are chert (a form of microcrystalline quartz), volcanic rocks and many others.
•Notable Occurrences include many Western USA localities; Mexico; Australia; England; Czech Republic and many other localities around the world.
•Best Field Indicators are color play & opalescence, low density, fluorescence, fracture filling tendency and lack of cleavage or crystal faces.
Fracture: Uneven to Conchoidal
Hardness: 6 Orthoclase
Luster: Pearly
Rock Type: Sedimentary
Specific Gravity: 2.68-2.72
Streak: White
Variety: Opal is a hydrated amorphous form of silica; its water content may range from 3% to 21% by weight, but is usually between 6% to 10%. Because of its amorphous character it is classed as a mineraloid, unlike the other crystalline forms of silica which are classed as minerals. It is deposited at a relatively low temperature and may occur in the fissures of almost any kind of rock, being most commonly found with limonite, sandstone, rhyolite, marl and basalt. Opal is the national gemstone of Australia, which produces 97% of the world's supply.[4] This includes the production of the state of South Australia, which amounts to around 80% of the world's supply.[5]
The internal structure of precious opal makes it diffract light; depending on the conditions in which it formed it can take on many colors. Precious opal ranges from clear through white, gray, red, orange, yellow, green, blue, magenta, rose, pink, slate, olive, brown, and black. Of these hues, the reds against black are the most rare, whereas white and greens are the most common. It varies in optical density from opaque to semi-transparent. For gemstone use, its natural color is often enhanced by placing thin layers of opal on a darker underlying stone, like basalt. Common opal, called "potch" by miners, does not show the display of color exhibited in precious opal. [6]Collection
RAGM Mineral CollectionAcquisition
Accession
2013.1Source or Donor
Crater Rock Museum (Misc. donors)Acquisition Method
GiftDimensions
Width
5 inDepth
3 inLength
5-1/4 inLocation
Location
Display Case
DS-10Room
Delmar Smith HallBuilding
Crater Rock MuseumCategory
ExhibitMoved By
Ian C.Date
January 22, 2025Notes
World case installationLocation
Display Case
FS-3Room
Frieda Smith HallBuilding
Crater Rock MuseumCategory
PermanentDate
August 18, 2023
