To make these varieties easier to describe in the field, I recommend classification of silica to include two varieties:
(1) quartz (macrocrystalline) and
(2) chalcedony (cryptocrystalline and microcrystalline).
Let's look at Quartz.
When found in veins (or in those nifty geodes), “quartz” can occur as individual crystals, tiny clusters of crystals (drusy quartz), or massive quartz with no apparent crystalline form. Quartz can be transparent, translucent or opaque. If vein quartz contains gold, we get excited. If barren, it is known as “bull quartz” which is massive, often milky white and of no economic value.
Some quartz veins may have uncommon visible gold filling fractures in the mineral. In other cases, the precious metal may be hidden in pockets of sulfide minerals (such as pyrite) in the vein. The gold can be visible or invisible, and will be unevenly distributed in veins.
Gold-bearing veins have ore grades that rarely average as high as 0.5 ounce per ton of gold (opt Au). Today, many gold mines produce ore at average grades of 0.15 to 0.015 opt Au. Veins with low grades must have considerable tonnage and other favorable characteristics to make them economic. The famous Homestake mine was considered to be rich. Yet during its early history, the ore averaged 0.3 opt Au. This grade gradually decreased to 0.15 opt Au until the mine closed when the mine operations reached depths below 8,000 feet. It just became uneconomic to haul the ore from such great depths to the earth's surface.
It is possible to recover high-grade hand-sized specimens enriched in gold, but these are restricted to small pockets in ore shoots. After searching for gold and other minerals for more than 3 decades, the highest assay I ever got was just over 4 opt Au with more than 11 opt silver at Mineral Hill in the Black Hills (Hausel and Hausel, 2011), and this was from a channel sample dug across the entire width of the vein (this property might make a very nice family-sized operation, but because of the nature of the deposit such as a horizontal veins, associated breccias in an alkalic intrusive, this property should be drilled to depth to see what lies below the surface 10 feet, 100 feet, 1000 feet and more).
If I wanted to be selective and produce an assay with a hundred or more ounces per ton, I could have taken a small, thumbnail-sized piece of visible gold with a little quartz from the channel sample and have had it assayed – but this would not have provided me with any scientific information. Still, many people high-grade samples to promote properties. And there are others who simply invent their gold assays.
High silver assays are more common simply because silver is more common than gold. There are galena-rich and tetrahedrite-rich silver veins in the Kirwin district in the Absaroka Mountains east of Yellowstone that were abandoned years ago. A few of these old mines yielded more than 100 opt silver from channel samples across veins taken by AMAX in the 1970s (Hausel and Hausel, 2011). Most of these veins would be considered economic at today’s silver prices if it wasn’t for the US Forest Service piecemeal withdrawing all of the potentially commercial mineral deposits in the Absaroka Mountains in Montana and Wyoming, effectively closing about 10,000 square miles. This region of northwestern Wyoming and southwestern Montana could easily have become one of the great gold-silver and porphyry copper-silver-gold districts of the West creating thousands of high-paying jobs and adding to our nations GNP rather than ending up in our growing national debt column.
Some veins have visible gold, and some have gold that is not visible to the naked eye let alone with a microscope. Such invisible or ‘noseeum’ gold can be detected by assay: some invisible gold occurs as gold atoms replacing individual atoms within the crystal lattice of a sulfide mineral, such as replacing some of the iron atoms in pyrite or copper and iron in chalcopyrite.
When invisible gold became a target of gold companies after gold prices rose to some of their highest levels in the 1980s, scam artists began promoting worthless properties as having invisible gold. These promoters often reported average gold ore to assay 10 to 100 opt of invisible gold with invisible platinum.
Auriferous (faulted) quartz vein in metatonalite in the Mary Ellen gold mine, South Pass, Wyoming.
Pyrite, or fool’s gold, can sometimes contain considerable gold within the crystal lattice. There are reports of pyrite with as much as 1,000 to 2,000 parts per million (ppm) gold (as much as 70 opt) hidden within pyrite. Exploration geologists often assay pyrite for gold because of this relationship and in some cases, I’ve seen prospectors crush massive pyrite to a fine greenish-black power and pan the powder for gold. The gold can occur as tiny inclusions in pyrite, or can substitute for iron in the atomic structure. In the latter case, one could only chemically detect the gold in assay and no amount of panning would help. To recover such gold, would require roasting and cyanide leaching.
Hexagonal quartz prismsfrom Hot Springs, Arkansas, capped by pyramids.
Quartz can occur as massive material and sometimes is found as excellent prismatic crystals. Less common are doubly terminated quartz crystals with pyramids at the terminations, such as those with a misnomer of “Herkimer diamonds” found in silicified dolomite at Little Falls, Herkimer County, New York.
Quartz crystals range from microscopic grains to specimens weighing hundreds of pounds. Some amethyst was found at Thunder Bay, Ontario that is as much as 10 inches in diameter.
Hardness. Hardness is often reported using Moh’s scale. This is a relative hardness scale used by geologists, rock hounds and mineralogists. On Moh’s scale, diamond has been assigned a hardness of 10 and graphite 1. Quartz was assigned a hardness of 7 and is softer than diamond, corundum (H=9) and topaz (H=8). But quartz is harder than your car’s windshield, which has hardness of 5.5 to 6. This is why your windshield gets pitted when you travel through Arizona’s sandstorms. Quartz will scratch and pit the softer windshield.
Color and Inclusions. Quartz occurs in a multitude of colors including colorless, white, red, orange, yellow, gray, brown, black, lavender, violet, purple, pink, blue and green. Essentially every color of the rainbow has been seen in quartz. In its chemically pure form, quartz is colorless to white. But like many gemstones, small amounts of chemical impurities give this mineral color.
Blue quartz from Montana.
Various valence states of iron produce many colors in quartz; the most desired by collectors is purple to red-violet of amethyst, and lemon-yellow of citrine. Iron is responsible for reds, yellows and browns in agates and gives carnelian and jasper their brownish-reds and orange colors. Iron and/or manganese will produce browns and blacks in dendritic agates, traces of titanium is responsible for the characteristic pink in rose quartz, and nickel gives chrysoprase its apple-green color.Fuchsitic (chrome-rich mica) quartz from Wyoming.
Mineral inclusions in coarsely crystalline quartz may impart colors as well as optical effects such as chatoyancy. Inclusions are simply tiny crystals enclosed by a larger crystal. Essentially all natural crystals have mineral inclusions – this is how I was able to show that the State of Wyoming was rewarding its employees with not-so-expensive pins presented to employees after serving 20 to 25 years. The lapel pins were touted as having valuable rubies. Being curious, I examined the tiny, faceted, rubies in the lapel pins and found they contained no mineral inclusions – thus they were synthetic rather than natural and probably cost less than a few dollars each as the rubies could easily been produced and faceted for <$1.00 for in India or Sri Lanka.
Reddish-brown hematite flakes and flakes of green fuchsite mica may yield a glittery metallic appearance (aventurescence) in some quartz. Rutilated quartz (quartz with rutile inclusions) may exhibit yellowish-brown to golden-yellow color, but also copper-red and silver-gray color. Although rutile in quartz is relatively common, rutile in highly transparent quartz is rare.
Gold has always been noted to have an affinity for quartz to such a degree prospectors almost always dig on quartz veins in a search for the precious metal. Striking specimens of gold-in-quartz have been recovered from the Badger mine, Mariposa County, California, the Sixteen-to-One mine in Sierra County, California, the Potato Patch in Arizona, and from several mines in Canada and Australia.
Milky quartz with native gold and mariposite mica from Ontario, Canada.
Gas-fluid inclusions are also found in natural quartz: most are microscopic. Analysis of gas-fluid inclusions provide a wealth of information about pressure, temperature, and chemical environment in which quartz crystallized. These inclusions are primarily formed of gaseous and liquid carbonic acids. Carbon dioxide, water, common salt, natural petroleum fluids, gases, and solids are found as inclusions. Doubly-terminate Herkimer quartz crystals have been known to host amorphous hydrocarbons as inclusions.
Specific Gravity. Quartz has a relatively low specific gravity (2.65). This indicates that even though it is more than twice as heavy as water, it is light enough that it can easily be washed out of a gold pan. Gold, however, has a specific gravity of 15 to 19.3, which is why gold will stay in a gold pan unless you’ve had too much coffee.
Cleavage. Quartz does not have cleavage. In other words, you are not going to break quartz along distinct planes like a diamond or ruby. If you strike quartz with a chisel to try to cleave, it will break into many fragments with conchoidal fracture (like broken glass).
Rock Crystal. Rock crystal is colorless quartz. Because of its low refractive index (refractive index is the degree to which light is bent to produce a rainbow) and glassy appearance, it is seldom used as a gemstone. It is unlike diamond which has high refractive index. Quartz exhibits characteristics similar to synthetic glass and has few properties that would make it attractive for adornment other than color. Some colorless raw quartz crystals have periodically been used in earrings and necklaces due to the characteristic hexagonal prisms that are visually interesting.
Amethyst. Amethyst is colored quartz that ranges from mauve to deep violet. The coloring agent for amethyst is small amounts of iron (Fe3+) distributed in the crystal structure in layers. Thus in many cut amethysts, clear bands of quartz may be seen alternating with colored bands. Amethyst has been one of the more popular gemstones throughout history and is even mentioned in the Bible as one of 12 sacred stones. Gems of similar color were once grouped together centuries ago, such as amethyst and sapphire, which is the origin of the term oriental amethyst that is sometimes applied to sapphire with similar color to amethyst.
Citrine. Citrine is a pale- to dark-yellow, brownish-yellow, or honey-yellow quartz with russet tint named after its resemblance to citrus lemon. It has been mistaken for topaz in the ancient past and erroneously given names such as ‘topaz quartz’. Even so, citrine can be distinguished in both faceted and natural form from topaz by refractive index and specific gravity. In hand specimen, crystal habit and cleavage can be used to distinguish topaz from quartz. The specific gravity for citrine (and feldspar gems) is the lowest for yellow, transparent gemstones, whereas topaz is noticeably higher. The luster is slightly inferior to topaz and topaz may show signs of incipient cleavage that would be non-existent in citrine. The coloring agent for citrine is iron. Most citrine used as gemstones are transparent to translucent.
Smoky Quartz. Smoky quartz varies from black to brown to smoky yellow and grades into citrine. The dark color of smoky quartz is thought to be from radioactive damage during exposure to radiation. Upon heating, smoky quartz will turn colorless, and return back to its smoky appearance with exposure to radiation. The best-known locality for smoky quartz is the Swiss Alps where veins have yielded many tons of beautiful crystals. Other notable localities are Russia, Brazil, Madagascar, and Scotland. In the US, smoky quartz has been reported in the Pikes Peak region of Colorado, and at various localities in Maine and New Hampshire.
Rose Quartz. Rose quartz often occurs as coarse-crystalline anhedral (formless) quartz that varies from pale pink to deep rose-red, which will fade with exposure to sunlight. Rose quartz is seldom transparent and instead is turbid. Its color is thought to be due to trace titanium. In some rose quartz, microscopic needles of rutile are found oriented in three directions at 120o from one another and at right angles to the c-axis of the crystal. When cut into a cabochon with proper orientation, such rose quartz will produce distinct 6-rayed stars due to light being reflected off the rutile mineral inclusions.
The color in rose quartz is a result of trace titanium.
Chatoyant Quartz. There are several minor ornamental stones of chatoyant quartz. This quartz contains parallel fibrous mineral inclusions that exhibit wavy reflections as they are rotated in light. Rather than fibrous mineral inclusions, others associate chatoyancy with thin hollow tubes known as etch tubes. Chatoyancy is also seen in Tiger’s Eye, a variety of cryptocrystalline quartz.