Crystal system: Hexagonal
Chemical formula/composition: Silicon dioxide (SiO2) for quartz and Hydrous silicon dioxide (SiO2. H2O)
Crystal habit: Quartz--typically 6-sided prisms terminating in a pyramidal point, twinning possible. Opal occurs as masses, amorphous habit, or pseudomorphing other minerals and fossils
Specific gravity: 2.65
Luster: glassy to vitreous, Opal is opalescent (a play of colors due to diffraction of light)
Toughness: good for quartz, less so for opal that tends to dry and fracture.
Optics: refractive index: 1.54 ND, Birefringence: low, Dispersion: 0.044 (extremely high)
Color: colorless, pale yellow (citrine), white (milky quartz), Purple (Amethyst), pink (Rose Quartz), gray or brown to black (Smoky Quartz) are also common. Cryptocrystalline quartz (also called chalcedony) (only microscopic crystals) varieties can be multicolored ,commonly red (jasper), black (onyx), orange (carnelian), blue, green (chryosprase). Opals are usually considered either common, which is translucent to opaque and usually brown to tan, or gem quality such as black, white, or fire (opal that is red to orange)
Other: U.V. fluorescence: commonly nonfluorescent, fluoresces blue, green, yellow or red; many opals are fluorescent under U.V.
Varieties: based on color as above
Localities: Worldwide, extremely common; the best black and white opal is from Australia.
Common simulants None for gems but for rock crystal glass under the name crystal. Opal is simulated by plastics and glasses such as Slocum stone.
Synthetics: Synthetic is known but rarely used for gems. Mostly important is synthetic opal (detailed subject descriptions under synthetics).
Quartz is one of the most variable and interesting of the semiprecious stones. Quartz never reaches a great value as a gemstone, but large and unusual natural specimens may be quite collectible and expensive.
Quartz lacks the optical properties to make an excellent gemstone. It is just hard enough to be worn in rings. Quartz has many colors, and both clear/transparent crystals and translucent to opaque cryptocrystalline (hidden or microscopic crystals in massive form) material are used for jewelry.
The most important crystalline varieties are:
Amethyst = purple
Citrine = yellow
Ametrine = a mixture of citrine and amethyst
Smoky = gray to black
Rose = pink
Milky = white
Crystal = colorless
Cryptocrystalline (literally secret crystals) varieties are made of aggregates of microscopic crystals and include:
Agate = a general name for cryptocrystalline varieties.
It often shows banding or inclusions such as moss agate
Aventurine = a green variety
Bloodstone = dark green with red (jasper) flecks
Chalcedony = agate (just another name of cryptocrystalline materials)
Chrysoprase = a rare green variety colored by the element nickel
Carnelian = various shades of orange (sometimes banded).
Jasper = a red variety
Onyx = bands that are white and black
Plasma = dull green (inclusions of the mineral chlorite)
Prase = leek green (inclusions of the mineral actinolite)
Sard = yellowish-red to brownish-red (inclusions of the mineral limonite)
Sardonyx = banded sard and white agate
These cryptocrystalline varieties often are porous enough to accept dyes, oils, and other chemicals and can be chemically stained colors that are either enhancements or purely artificial.
There are also varieties showing special features, such as chatoyancy,
which means "cat's eye” in French, and asterism which is a six-rayed star.
Chatoyancy is a shimmering effect caused by fibrous structure or parallel channels in a mineral. It is at right angles to the fibrous structures. A similar effect is seen in silk and other shiny fabric.
Quartz cat's eye--(regular cat’s eye is chrysoberyl) has an eye effect.
Hawk’s eye--similar to tiger’s eye but in a gray color
Tiger's Eye--a chatoyancy in shades of brown and yellow resembling the stripes of a tiger.
Asterism is cause by inclusions that follow crystallographic axes and align at 60 degrees to each other to create a star. As the stone is moved the star’s rays move. The rays appear white. Most typically, rose quartz shows asterism, but it is never as strong as in sapphire or ruby.
Pseudomorphs–quartz may replace other substance such as wood
or shells, creating petrified wood or silicified fossils. Both are
of interest to collectors. Because the quartz replaces and is not
the original material, the terms “pseudo” (false) & “morph” (shape)
Though not a kind of quartz opal is made of the same chemicals. Silicon and oxygen are combined to form SiO2 with additional water. The silicon dioxide is hydrated and forms microscopic spheres of opal. It often forms by hydrothermal processes: hot water circulating up into colder rocks and then precipitating minerals on cooling. Opal fills fractures and replaces soluble substances such as fossils.
Its properties are very different than those of quartz. It tends to form microscopic spheres that accumulate to form the opal and that cause a diffraction of light. The diffraction of light creates opals most important characteristic: a play of colors. The play of colors or opalescence is a many hued sparkle that demands attention. The clear and bright Newtonian colors caused by the diffraction are exciting. Perhaps only bioluminescent creatures of the deep sea or peacock feathers come close.
There are 3 main varieties of opal:
1) Black opal–the most valuable. It usually is a dark blue
background with colors very obvious (the photo below is a mixture of black
and white opal).
2) White opal–the runner-up. Its white background can vary from bright to cream.
3) Fire opal–which lacks the play of colors compared to the two above is much less valuable.
The first two varieties are almost exclusively cut en cabochon. Fire opal can be cabochoned or faceted.
All 3 varieties are sensitive to drying (desiccating). Remember that silica is used as a desiccant and can dry out things. So opal should be soaked occasionally to avoid drying and cracking. In the same way, since opal absorbs moisture, dirt can infiltrate the crystal causing a problem. Opals should not come in contact with dirty dishwater for instance.
Opal can also be harmed by bumps; it is much less durable (low tenacity) than quartz; excessive heat can damage it (no steam cleaning), and so can ultrasonic cleaning.
Overall, since it is vulnerable to damage in many ways, opal should not be used in rings, but is best for pendants and earrings.
Opal is usually imitated using a glass with colored foil embedded in it, called slocum stone. Opal is also sandwiched between pieces of glass or quartz and can be backed with stone. This creates doublets and triplets.
Doublets and triplets are actually perfectly legitimate, unless misrepresented as pure stones. This fakery does occur. Both doublets and triplets may protect thin slices of opal from damage. Doublets and triplets should not be soaked in water as cements may be damaged.
One bit of trickery exists that is worth mentioning. Not all black opals started their history as black opals, some have been sugared. Sugaring is a way of adding carbon black into a less valuable white opal. The stone is soaked in a sugar solution and then exposed to acid that burns the sugar creating a black background for the opal. Another trick is to back a thin slice of light-colored opal onto a dark background stone and create a black opal in this manner. These stones can be difficult to detect and are a threat to the legitimate market.
Opal has a legend that states it brings bad luck to the wearer. Australia, the world's largest producer, has fought this myth for the last century and has pretty much won the battle. However, even though opal has some properties that border on those of the more valuable gem materials and exceeds them in their colorfulness, opal is limited by its durability and will never reach the level of the big 4, emeralds, diamonds, rubies, and sapphires.
Though certainly good for a lifetime or two delicate opals will not
hold up for billions of years the way diamonds will.