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. 

Elimia tenera: A commonly misidentified Eocene freshwater snail

Over the years, as I have viewed various collections of fossils, I have come across specimens of a small fossil gastropod that occur in great abundance. Rocks containing these shells can be found for sale in rock shops or online, and the shells are commonly and incorrectly called “Turritella agate.” These rocks do not consist of Turritella; rather they consists of specimens of the freshwater gastropod Elimia tenera (Hall, 1845), which have been preserved in chalcedony. Actual specimens of Turritella can be much larger, possess only spiral ribs, and are known only from shallow-marine deposits. Elimia tenera has both radial and spiral ribs, and the aperture of Elimia is quite unlike that of Turritella.

This rock (77 mm width) is fully packed with specimens of E. tenera.

This polished slab (37 mm width) shows only the
cross section of shells of E. tenera.

These three specimens of E. tenera are internal casts (i.e., each one shows
only the interior of a shell, which was filled with chalcedony).
The largest specimen is 14 mm height. 

There has been considerable disagreement in blogs and websites as to whether or not the E. tenera specimens, found in rock shops, have been replaced by chalcedony or agate. Technically speaking, chalcedony is the “culprit.” It is a microcrystalline form of silica, and chalcedony has many varieties, including agate, which commonly has multi-colored curved or angular banding. The specimens of E. tenera that I have seen were replaced by a fairly uniform brown or gray color of “ordinary” chalcedony and not replaced by the more eye-catching, beautiful colors typically associated with agate.

Elimia tenera: Specimen on the left (19 mm height) shows the spiral ribs, and the
specimen on the right (14 mm height) shows both spiral and radial ribs.

Elimia tenera, which used to be (and incorrectly) called Goniobasis tenera, is a freshwater snail that lived in shallow subtropical lakes with intermittent volcanic eruptions nearby. This gastropod is of Eocene age and is commonly found in the Laney Member of the Green River Formation in Utah. This is the same formation that famously has many very well preserved fish, insect, leaf, and other fossils.

Genus Elimia belongs to family Pleuroceridae, and, as currently defined, this family  today is confined entirely to North American fresh waters: The eastern United States and into Texas and from southern Canada to Florida. Pleurocerids might be relicts (“living fossils”) from earlier geologic times (Paleozoic?) in eastern North America.