Agate Facts

Agate has a seven Mohs hardness, making it desirable in the lapidary trade. Agate is translucent and exhibits a vitreous luster. As with other varieties of quartz, its chemical formula is SiO2. The name agate comes from the river Achates in Sicily and was given that name by the Greek philosopher, Theophrastus. Agate names can be derived from the location where they are found such as Botswana agate or Laguna agate or because of a feature they exhibit like lace, fire or moss.

How Agates Are Formed

Agate formation takes place when silica solutions fill voids in cavities layer by layer such as in Florida’s agatized coral. The voids typically are in solidified volcanic lava flows and the layering over long periods of time can result in different colors because of chemical changes in the silica solutions. The layering can deposit the chalcedony in concentric circles around the wall of the cavity or build the layers up from the bottom of the cavity.

Decorative Agates

Agate is used to produce cabochons, beads, and free-form objects in jewelry making. Book-ends, ashtrays, and sculptures are also made using agate.

Positively Agate

In the metaphysical realm, agate is said to change negative energy into positive energy and to heal anxiety and internal anger. Agate may also help with concentration and boost mental function.

Where to Find Agates

In the United States, agates are found in Oregon, Wyoming, Montana, Arizona, Idaho, California, Washington, New Mexico, Iowa, Wisconsin, Michigan, Minnesota, Utah, Florida, Colorado, Arkansas, and Nevada. Around the world, they are found in Australia, Germany, Brazil, Czechia, Botswana, Mexico, Morocco, Afghanistan, Argentina, Canada, Chile, India, China, and over 40 other countries.

This agate field guide appeared in Rock & Gem magazine. Click here to subscribe. Story by Richard Gross.

The post A Field Guide to Agate first appeared on Rock & Gem Magazine.

These Mexican geodes are different than their cousins, Trancas geodes. They are found in one small area just slightly north and east of Chihuahua City and are known in the trade as “Mexican Coconuts.” Collectible in this case does not mean field collectible as production is pretty much confined to one area owned by one family.

Unlike giant geodes, Las Choyas geodes for sale in rock shops and gem and mineral shows are between 2.5 and 3.5 inches in diameter although they can range up to six inches in diameter. Almost all are spherical in shape thus affirming our concept of what shape a geode should have.

Because the opportunity for field collecting is not available to the general public, most hobbyists will usually only have a few Las Choyas geodes in their collection.

The word geode comes from Ancient Greek and means “earthlike.” For the metaphysical believer, geodes, like Las Choyas geodes, are said to bring flow, harmony and resilience to the bearer.

How They Are Formed

Unlike the famous Keokuk geodes that form in a sedimentary environment, Las Choyas geodes form in an igneous environment. This environment comes about as a result of a volcanic eruption producing an ash flow that contains bubbles that are trapped in the rhyolitic tuff rock formation when it solidifies.

These hollows in the rock provide the chamber where the mineral-rich fluids flow to start the process of geode formation. The total process may take millions of years and involves a complex series of geological events that ultimately yield these spectacular geodes.

The actual process of growth and development of these Las Choyas geodes is complex. Chalcedony rind formation is common, but it is not unique. In some of these geodes, siderite (FeCO₃) forms the rind.

While quartz (SiO₂) and calcite (CaCO₃) are the most common minerals, 18 different minerals have been identified in Las Choyas geodes. Most of these minerals are very rare with a few only found using a scanning electron microscope!

Minerals Commonly Found in Las Choyas Geodes

This list contains the minerals that the geode hobbyist might visibly encounter in their Las Choyas collection. The remaining minerals found in the Las Choyas geodes are either too small or rare that they will not be encountered by the geode hobbyist.

Las Choyas geodes can be hollow, semi-hollow, or completely filled and solid.

• Quartz (SiO₂) may be found in the chalcedony rind, as a distinct band of crystals around the cavity, as a drusy layer, or as an over-growth on other minerals. The crystals are clear, smoky or amethyst.
• Calcite (CaCO₃) may appear in different crystal forms such as pseudocubic, dogtooth, nailhead, rhombic, and sheets.
• Siderite (FeCO₃) forms the rim in some of the Las Choyas geodes as well as appearing in the cavity.
• Goethite (HFeO₂) appears as small black needles with chisel-like terminations in these geodes.
• Hematite (Fe2O₃) appears as small black, brown, or red spheres sometimes on or included in quartz.
• Mordenite [(Ca, Na₂, K2) Al₂Si₁₀O₂₄.7H₂O] forms a mat of white fibers and may fill the cavity of the geode.
• Selenite (CaSO₄.2H₂O) This crystal form of gypsum may appear as small clear tabular crystals.
• Pyrolusite (MnO₂) This manganese oxide forms dendritic crystals that can grow around and on the quartz and calcite crystals.
• Ramsdellite (MnO₂) This manganese oxide forms small black crystal groups with needle-like crystal terminations.
• Todorokite [(Mn, Ca, Mg) Mn₃O₇H₂)] forms black mats of fibrous crystals.

Mining Las Choyas Geodes

Las Choyas geodes are mined in a desolate area of the Chihuahuan desert. Mining involves extreme working conditions and very hard labor. The geode-bearing zone tips down from the surface at a 10-degree angle.

This means that as each area is exhausted of geodes, one has to go deeper underground. Going underground in this formation means drilling and hand-finishing a three-foot diameter hole through rhyolitic ash tuff rock.

After reaching the geode-bearing zone, miners must carve out open areas to remove the geodes using picks and hand geology hammers. The depth where the geodes are mined is now over 125 feet below the surface.

About 80% of the mined geodes turn out to be solid. While not as valuable as the hollow ones, they do have a use in the lapidary trade as material for cabochon jewelry and decorative objects such as bookends.

This story about Las Choyas geodes previously appeared in Rock & Gem magazine. Click here to subscribe. Story by Richard Gross.

The post Las Choyas Geodes 101 first appeared on Rock & Gem Magazine.

Unique Geodes With Crystals

Trancas geodes are generally small – just two to four inches in size. They are not very colorful with only pale colors typically ranging from gray to blue, but they make up for a lack of color because they often produce fabulous drusy quartz centers.

The centers usually contain clear quartz, chalcedony, calcite, aragonite and sometimes quartz scepters. Trancas geodes may also contain a form of quartz that is rather hard to categorize.

It might be best described as “curved finger-like projections.” Some have described Trancas geodes as miniature caves with stalactites and stalagmites lining the hollow interior. Trancas geodes have a thin outer shell that is usually gray, brown, cream or peach colored.

A fun bonus with Trancas geodes is that they tend to fluoresce green under short-wave UV light because many contain a small amount (not harmful) of uranium.

While most contain hollow centers, some Trancas geodes present as thundereggs with a completely solid or nearly solid agate center. This is not considered as desirable as the drusy quartz centers.

Where to Find Trancas Geodes?

Trancas geodes are normally found during surface mining activities.

Breaking Barriers

In the metaphysical world, geodes are thought to help with overcoming barriers and obstacles. They are thought to help with being calm and centered. The crystals inside a geode are also thought to work together to provide assistance…kind of a two-for-one!

Geodes with Crystals Facts

The word geode comes from the Greek word Geoides which means “earthlike.” Geodes are secondary structures formed in sedimentary and volcanic rocks.

This story about Trancas geodes appeared in Rock & Gem magazine. Click here to subscribe. Story by Richard Gross.

The post Trancas Geodes with Crystals first appeared on Rock & Gem Magazine.

Quartz occurs in many forms in the crystalline and microcrystalline states. Both of these states contribute many specimens that are used to produce jewelry. Quartz can be found in most sites where minerals are found. Crystalline Quartz also known as rock crystal, or crystalline quartz has a Mohs hardness of seven which makes it attractive to the jewelry trade. On the Mohs scale, minerals can only be scratched by those with a higher number meaning quartz can only be scratched by minerals with a rating of eight (topaz), nine (corundum), or ten (diamond).

Rock crystal has a glasslike, or vitreous luster and occurs as water-clear crystals, purple, yellow, and other colors when it has certain inclusions. Quartz crystals form in the hexagonal system. Depending on the environment, they can form many variations. They can be doubly terminated, drusy gemstones, twinned, and can form scepters. They can even form irregular finger-like projections, as found in some Trancas geodes with crystals. Quartz crystals have a specific gravity of 2.7 and exhibit conchoidal fracture

Crystalline Quartz Uses

Crystalline Quartz uses include: jewelry, watches and  clocks, heat-ray lamps, countertops, prisms and electrical equipment.

Healing Quartz

The word quartz comes from the German word quartz. In holistic practices, quartz is considered to be a powerful healing stone. It is thought to bring stability and harmony and aid in concentration. Quartz (SiO₂) is a common silicate mineral. Other minerals in the silica group include chalcedony, opal, and garnet. Silicates make up a quarter of all known minerals with quartz being an important rock-forming mineral. Quartz is usually categorized as the second most prevalent mineral in the continental crust with feldspar listed as the most prevalent.

Quartz occurs in many forms in the crystalline and microcrystalline states. Both of these states contribute many specimens that are used to produce jewelry. Quartz can be found in most sites where minerals are found.

How to Identify Quartz – Properties

Also known as rock crystal, crystalline quartz has a Mohs hardness of seven which makes it attractive to the jewelry trade. On the Mohs scale, minerals can only be scratched by those with a higher number meaning quartz can only be scratched by minerals with a rating of eight (topaz), nine (corundum), or ten (diamond).

Rock crystal has a glasslike, or vitreous luster and occurs as water-clear crystals, purple, yellow, and other colors when it has certain inclusions. Quartz crystals form in the hexagonal system. Depending on the environment, they can form many variations. They can be doubly terminated, drusy, twinned, and can form scepters. They can even form irregular finger-like projections, as found in some Trancas geodes. Quartz crystals have a specific gravity of 2.7 and exhibit conchoidal fracture.

This story about how to identify quartz cutting previously appeared in Rock & Gem magazine. Click here to subscribe. Story by Richard Gross & Pam Freeman. 

The post How to Identify Quartz first appeared on Rock & Gem Magazine.

Crystal Formation & Properties

Calcite crystals form in the trigonal system and their habits vary more than any other mineral. Common forms include dogtooth, tabular, needle-like, pseudocubic and prismatic. With a Mohs hardness of three, rhombohedral cleavage and bubbling in contact with acid, calcite is easily identified.

Calcite dissolves in acid — almost no other minerals do — and can be useful when calcite entombs a more desirable mineral. Soaking the specimen in acid will dissolve the calcite and expose the desirable mineral. This is a common technique used by geode collectors.

Calcites’ colors include white, clear, gray, yellow, green, blue, brown, and black. Some calcite will fluoresce under both short-wave and long-wave UV light. Calcite may be found in caves as stalactites or stalagmites. Calcite and aragonite are polymorphs as they have the same chemical composition (CaCO3) but have different crystal structures.

Calming Calcite

For those that perceive calcite as having healing powers, calcite is said to have the power to cleanse energy, enhance spiritual growth, balance chakras, absorb negative energy and calm the mind.

Calcite Identification

There are over 300 forms of calcite, some more popular than others. Gem-quality calcite is often translucent and is rarer than other mineral specimens. It is colorless, white, gray, yellow, pink, or green. Iceland spar is a clear, colorless, transparent variety of calcite that exhibits strong double refraction or birefringence. It is said the Vikings used Iceland’s spar for navigation.

Calcite is also a component in many rocks such as limestone, marble, and carbonatite. Its name comes from the Greek word calyx which translates to “lime” and relates to being a component in limestone. Calcite is also the mineral component of marble used in sculpture and decorative household use. Carbonatite is important with many uses such as host rocks for fluorspar, rare earth elements, apatite, and vermiculite, as well as a source for calcite.

Countries of Origin

Afghanistan; Argentina; Australia; Austria; Belgium; Bolivia; Brazil; Bulgaria; Canada; China; Colombia; Congo; France; Germany; Greece; Iceland; India; Ireland; Italy; Kosovo; Latvia; Madagascar; Mexico; Morocco; Namibia; Norway; Pakistan; Peru; Portugal; Russia; Slovakia; South Africa; Spain; Sweden; Switzerland; Tanzania; United Kingdom; United States

This story about calcite identification previously appeared in Rock & Gem magazine. Click here to subscribe. Story by Richard Gross and Pam Freeman. Photos by Richard Gross.

The post A Calcite Identification Guide first appeared on Rock & Gem Magazine.

I first saw it at the Tucson Show in 1994. It was mined by Bryan Lees of Collector’s Edge. The King is in the Denver Museum of Nature and Science courtesy of the Coors Family. The Alma Queen, after passing through several hands, is now in the Houston Museum.

A Superb Specimen

Unlike the King with its single large rhodochrosite crystal, the Alma Rose is blessed with a cluster of sharp three-inch rhombs of very fine color interlocked to form a grouping of crystals some six inches across.

Each rhodochrosite is individual and sharp enough so each can be enjoyed, yet the cluster is undamaged and positioned so it is a wonderful example of the rhombic form of this manganese carbonate.

As if the cluster of crystals is not enough, a single rhodochrosite rhomb of matching size and color sits nearby as if refusing to join the cluster. This single crystal simply adds to the overall beauty of the specimen.

The rock matrix is completely covered and hidden by bright small quartz crystals.

Among all the quartz crystals are small contrasting dark sulfide crystals and a scattering of small colorful fluorite cubes.

To add a finishing touch, the final solutions released a scattering of small crystallized fine pale yellow stibnite sprays which developed randomly on and around the rhodochrosite. These contrast with but do not detract from the overall beauty of the specimen.

Delicate Digging

Just how such a perfect undamaged specimen can be extracted from its rock prison without damage is strong evidence of how far we have progressed in specimen recovery these last decades.

As an early collector, my tools included hammers and chisels, pry bars, and wedges. If you found a fine specimen on the exposed side of a vein opening, the difficulty to extract an undamaged specimen was obvious.

Today, with modern tools, such removal of the Alma King and Alma Rose was accomplished without serious damage to any of the exposed crystals.

The Alma Rose was on a solid rock wall, one side of a crystallized vein.

Removing the Rose

To remove such a beauty undamaged, miners employed power drills, diamond saws and hydraulic wedges. Keep in mind any serious vibrations of the rock holding the crystals would be fatal since rhodochrosite has perfect cleavage and any stress might cause crystals to part. This is what happened to the Alma King before it was ever exposed.

It was very lucky to have dropped in the vein untouched.

For the Alma Rose, drilling had to start well away from the open vein, which was first covered to protect it. Holes were drilled all around the wall section encircling the open vein. These served as a guide for the diamond saws that cut deeply into the wall rock. Slowly, the cut openings could be wedged open and more cutting was done to get behind the vein.

Once the entire vein section had been undercut it was broken loose from the wall with its still intact crystals. It could be carefully removed and taken to the specimen preparation lab at Collector’s Edge where trimming excess rock was slowly done, reducing the specimen to a manageable size and weight. Further trimming and shaping were done until the surface was exposed and crystals could be seen in all their beauty. With proper cleaning and shaping the Alma Rose was ready to be displayed.

This story about how rare is rhodochrosite appeared in Rock & Gem magazine. Click here to subscribe. Story by Bob Jones.

The post How Rare is Rhodochrosite? first appeared on Rock & Gem Magazine.