Varieties of Quartz: Part 2

The previous post showed some common varieties of macrocrystalline (visible crystals) of quartz (silica = SiO2). This post shows some common varieties of quartz that do not show any visible crystals and have a dense structure, referred to as  microcrystalline  [= cryptocrystalline] quartz. Two main groups are shown here: chalcedony and opal.

Chalcedony is the generic named given to materials that are composed of microcrystalline quartz. Chalcedony includes blue chalcedony, chert, "touchstone," agate, jasper, and "tiger eye." Examples of these are shown below: 

height 5 cm, from Nsanje District, Malaw (southeast Africa) 

Blue chalcedony (commonly called blue "quartz"). Chalcedony is a microcrystalline form of quartz (silica, SiO2), composed of intergrowths of quartz and moganite, and each has a different crystalline structure. 

height 3 cm

Chert is a hard, opaque chalcedony with an amorphous microcrystalline texture. Chert has conchoidal fracture, which produces very sharp edges, especially in flint (black or brown forms of chert). That is why chert was commonly used for making arrowheads.

height 7 cm

The image above shows a sedimentary rock consisting of chert layers (in black) alternating with impure white siliceous (silica bearing) material, referred to by some geologists as "porcelaneous chert" (i.e., resembles glazed porcelain).

height 7.5 cm

"Touchstone" is a black, nodular version of chert. For at least 2,500 years, it has been used by goldsmiths to test the purity of gold and silver by the streak left on the stone when rubbed against these metals. "Touchstone" is also called the "lydite stone," also spelled as "lyddite."

sliced and polished specimens, largest one height 7.5 cm

Agate is banded chalcedony and has a wide range of colors. The bands form over time and can have different colors, depending on the impurities. Agates form commonly in areas of volcanic activity or in limestone where waters, rich in dissolved silica flow through fractures and cavities in these rocks.

height 1.7 cm

An agatized snail, whose hollow interior was filled with agate.

petrified wood (14 cm width), from Petrified Forest, Arizona

Petrified wood is, in most cases, wood replaced by chalcedony. Other minerals (like opal), however, are also known to replace wood.

height 5 cm

Jasper is an opaque aggregate of chalcedony and microgranular quartz. Jasper is an impure variety of silica, thus it can have red, yellow, brown, green, or other colors. The red color of jasper is because of iron inclusions.

polished specimen, height 3.7 cm

"Tiger eye" is a metamorphic rock consisting of chalcedony that formed by replacement of an asbestos-group mineral (called crocidolite). "Tiger eye" has a golden brown to very dark brown color and silky luster. 

The other group of microcrystalline silica is opal, which is a mineraloid consisting of hydrated amorphous silica (water content 3 to 21 percent by weight; commonly 6 to 9 percent). Opal is deposited at relatively low temperature, and it can be deposited in the fissures of any kind of rock, especially volcanic rock. Opal is the national gemstone of Australia.

Examples of "precious opal" are shown in the next two images below. "Precious opal" displays flashes of colors when exposed to light (= a form of iridescence). 

height 1 cm

height 2 cm

height 4.5 cm

This last variety of opal is "fire opal," which is bright red, yellow, 
or orange in color.

width 10 cm

Both macrocrystalline and microcrystalline quartz occur commonly in geodes, which are round rocks with a hollow cavity line with crystals. In the example above, the geode has two hollow cavities, both partly filled with druzy (macrocrystalline) quartz alternating with layers of chalcedony.

width 12 cm

Cavities completely filled with chalcedony (e.g., agate, jasper) are called nodules. In the nodule shown above, macrocrystalline amethyst mostly filled a geode. The remaining cavity was filled eventually filled completely with chalcedony. This unusual specimen was collected by my friend, the late George Edward Davis.