Quartz and Silica Minerals Gallery

Different Kinds of Quartz

Left to right, rose quartz, amethyst, and rutilated quartz display some of this mineral's variety.

Doubly Terminated Quartz Crystal

Double-ended "Herkimer diamond" quartz crystals are found in a few places, but quartz is almost always attached at one end.

"Herkimer diamonds" are distinctive doubly terminated crystals of quartz from the Cambrian limestones near the town of Herkimer, New York. This specimen came from the Herkimer Diamond Mine, and they can also be found at the Crystal Grove Mine.

Bubbles and black organic inclusions are common in these crystals. Inclusions make a stone worthless as a gem, but they are valuable scientifically, being samples of the fluids that circulated in the rocks at the time the crystals were being formed.

It's a real thrill to dig for Herkimer diamonds, no matter what age you are. And studying the faces and angles of the crystals will give you an appreciation of their appeal to mystics and to scientists, both of whom take crystal form as a tantalizing clue to the true nature of matter.

Quartz Spears

Quartz crystals generally terminate in blades, not true points. Many pointed rock-shop "crystals" are cut and polished quartz.

Grooves on Quartz Crystal

A sure sign of quartz is these grooves across the crystal faces.

Quartz in Granite

Quartz (gray) breaks with a conchoidal fracture, making it glitter, whereas feldspar (white) cleaves along crystal planes, making it flash.

Milky Quartz Clast

Quartz is often milky like this pebble, probably an eroded chunk of a quartz vein. Its tightly interlocked grains do not have the outer form of crystals.

Rose Quartz

Rose quartz is milky quartz of a pink color, thought to be due to titanium, iron or manganese impurities or microscopic inclusions of other minerals.

Amethyst

Amethyst, the purple variety of quartz, gets its color from iron atoms in the crystal matrix plus the presence of "holes," where atoms are missing.

Cairngorm

Cairngorm, named for a Scottish locality, is the dark brown variety of smoky quartz. Its color is due to missing electrons, or holes, plus a whisper of aluminum.

Quartz in Geode

Quartz commonly forms a crust of crystals on the inside of geodes in addition to the layers of chalcedony (cryptocrystalline quartz) in this cut section.

Chalcedony in a Thunder Egg

The core of this thunder egg is composed of chalcedony (kal-SED-a-nee), the microcrystalline form of silica. This is about as clear as chalcedony gets. (more below)

Chalcedony is the special name for quartz with microscopically small crystals. Unlike quartz, chalcedony does not look clear and glassy but translucent and waxy; like quartz it is hardness 7 on the Mohs scaleor just a little softer. Unlike quartz it can take on every color imaginable. An even more general term, encompassing quartz, chalcedony and opal, is silica, the compound silicon dioxide (SiO₂). Chalcedony may contain a small amount of water.

The major rock type that is defined by the presence of chalcedony is chert. Chalcedony also very commonly occurs as a mineral filling veins and openings, like geodes and this thunder egg.

Jasper

Jasper is a red, iron-rich chert that is rich in chalcedony. Many varieties are named; this is "poppy jasper" from Morgan Hill, California. (click full size)

Carnelian

Carnelian is a red, translucent variety of chalcedony. Its color, like that of jasper, is due to iron impurities. This specimen is from Iran.

Agate

Agate is a rock (and a gemstone) composed chiefly of chalcedony. This is a particularly refined specimen from Indonesia. (more below)

Agate is the same kind of rock as chert, but in a much purer, more transparent form. It consists of amorphous or cryptocrystalline silica, the mineral chalcedony. Agate forms from solutions of silica at relatively shallow depths and low temperatures, and is exquisitely sensitive to the physical and chemical conditions around it. It is commonly associated with the silica mineral opal. Fossilization, soil formation, and alteration of existing rock can all create agate.

Agate occurs in infinite variety and is a favorite material among lapidaries. Its fluid forms lend themselves to attractive cabochons and similar flat or rounded jewelry formats.

Agate may have several different names, including carnelian, catseye and many fanciful names suggested by the shapes and colors of a particular occurrence.

This stone, magnified several times, displays cracks that extend only a few millimeters from the surface. They are completely healed and do not affect the stone's strength. For a larger specimen, see the agatized tree-trunk in the Fossil Wood Gallery.

For deep geologic information on agates, including hundreds of pictures, visit the Agate Resources page from the University of Nebraska. Agate is the state rock or state gemstone of Florida, Kentucky, Louisiana, Maryland, Minnesota, Montana, Nebraska and North Dakota.

Cat's-Eye Agate

Microscopic fibers of the amphibole mineral riebekite in this chalcedony specimen produce the optical effect called chatoyancy.

Opal, Hydrated Silica

Opal combines silica and water in a nearly random molecular structure. Most opal is plain and translucent or milky, but gem opal displays schiller. (more below)

Opal is a delicate mineraloid, hydrated silica or amorphous quartz. The mineral includes a fairly large amount of water molecules, and opals should not be left in direct sunlight or high temperatures.

Opal is a lot more common than people think, but it's usually a thin whitish film that lines fractures in rocks subjected to diagenesis or very mild metamorphism. Opal is commonly found with agate, which is cryptocrystalline quartz. Sometimes it is a bit thicker and has some internal structure that produces the highlights and color range of gem opal. This spectacular example of black opal is from Australia, where nearly all of the world's supply is mined.

The colors of gem opal arise as light diffracts in the ghostly internal structure of the material. The background layer, or potch, behind the colorful part of the opal is important too. The black potch of this black opal makes the colors appear especially strong. More typically, opal has a white potch, translucent potch (crystal opal) or clear potch (jelly opal).

Other Diagenetic Minerals