Acanthite and Argentite are usually grouped together in mineral guides as one mineral...  More

Acanthite and Argentite are usually grouped together in mineral guides as one mineral. However, they are scientifically categorized as two unique minerals, even though they are composed of the same substance. They only differ in crystal structure. As explained below, Argentite cannot exist at normal temperatures. Specimens labeled as Argentite are sometimes sold by dealers, however, these specimens are really Acanthite pseudomorphs after Argentite.

Because Acanthite is a significant ore of silver, and occurs in odd and unique forms, its specimens command high prices and are sought after by collectors.
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Actinolite and Tremolite are two very similar minerals that form a series with each o...  More

Actinolite and Tremolite are two very similar minerals that form a series with each other and essentially share the same chemical formula. Actinolite has a greater presence of iron over magnesium, whereas Tremolite has a greater presence of magnesium over iron.

The green color of Actinolite is produced by the iron in its chemical structure. The amount of iron varies among specimens, causing different contrasts of green. More iron will give a specimen a darker color.

Actinolite sometimes occurs as thin, splintery inclusions inside Quartz crystals. If the inclusions are parallel and dense, it will cause chatoyancy in the Quartz.

Actinolite and Tremolite share several recognized varieties. Mountain Leather, a thickly fibrous and leathery variety, has a silky luster, a soft felt-like feel, and elastic fibers. Nephrite, another fibrous variety, is made up of tough, interlocking fibers, so dense that the fibers are not discernible. Translucent Nephrite with a uniform or interesting color distribution that is extremely tough fits under the category of Jade.

Actinolite and Tremolite both contain a form of asbestos which is made of movable and elastic fibers. Actinolite asbestos is less common; most forms are in fact Tremolite. This form of the mineral contains significant health hazards and is further discussed in the Tremolite mineral detail page.
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Adamite occurs in many different lively colors. Different impurities are responsible ...  More

Adamite occurs in many different lively colors. Different impurities are responsible for the color types. When copper is present in Adamite, this gives a vibrant green or blue color, and when cobalt is present, it gives a pink to purple color. Some impurities such as copper prevent the Adamite from its habit of being fluorescent.

Adamite crystals are usually embedded in rusting, crumbly, brown Limonite matrix that stain the hands. Since the Limonite crumbles when scrubbed, care must be exercised when washing such specimens.
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Aegirine is a member of the pyroxene group , and forms a series with the mineral Augi...  More

Aegirine is a member of the pyroxene group, and forms a series with the mineral Augite. Aegerine is well known for its long slender crystals with very distinctive terminations, and some of the more lustrous forms of this mineral are true classics.
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Afghanite is a rare and coveted mineral with a striking blue color, first identified ...  More

Afghanite is a rare and coveted mineral with a striking blue color, first identified in 1968. With a very close resemblance to Lazurite, and occurring together in the same deposits, Afghanite is distinguished by its unique crystal habit. Afghanite is named after its most significant locality in northern Afghanistan.
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Since most forms of Agate are dull and uninteresting until they are cut and polished, we will show images mostly from the cut and polished forms for visual clarity.

Agate is the banded form of the mineral Chalcedony , which is a microcrystalline vari...  More

Agate is the banded form of the mineral Chalcedony, which is a microcrystalline variety of Quartz. Agate is the most varied and popular type of Chalcedony, having many varieties on its own. Although the pattern on every Agate is unique, the locality of an Agate will provide resemblances in banding style and color, thus lending many Agates with a geographic prefix. Some examples are Laguna Agate (named after Ojo Laguna, Mexico) or Botswana Agate (after the African country of Botswana). Other variety names used connote specific colors or patterns, such as Fire Agate or Eye Agate.

Agate usually forms in rounded nodules or knobs which need to be sliced open to bring out the internal pattern hidden in the stone. Most Agate is ugly in its natural state; specimens must be polished to bring out their full beauty. Much of the Agate sold to collectors has been treated, in the form of tumbled stones or polished slabs. Popular collector forms of Agate include nodules or geodes sliced in the middle into two polished cross-sections, or thin slabs from nodule or geode cross-sections.

The formation of Agate is most often from deposition of layers of silica filling voids in volcanic vesicles or other cavities. The layers form in stages with some of new layers providing an alternating color. Since the cavities are irregularly and uniquely shaped, each Agate forms its own pattern based on the original cavity shape. When a cavity is completely filled, it forms a solid mass of Agate, but often it is only partially filled, leaving a hollow void which often has crystalline Quartz growths on its innermost layer. This is the cause of Agate forming the outer lining of most geodes.

Agate is often dyed to enhance its colors. This is especially true of Agate from Brazil. Bright neon colors such as bright blue and red are rarely natural.

Agate is named after the Achates River (now known as the Dirillo River) on the island of Sicily, Italy, whose upper waters were an ancient source of this gemstone.

For additional information, see the gemstone section on Agate.
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Albite belongs to the Plagioclase Feldspar group, an isomorphous solid solution serie...  More

Albite belongs to the Plagioclase Feldspar group, an isomorphous solid solution series. Albite is one end member, containing sodium and no calcium. The other end member, Anorthite, contains calcium and no sodium. The intermediary members are Oligoclase, Andesine, Labradorite, and Bytownite. Oligoclase and Andesine are considered by some to be a variety of Albite rather then a separate mineral. The acclaimed Dana's System of Mineralogy lists these intermediary members as individual minerals, whereas the IMA does not recognize them as individual mineral species.

Albite also forms a series with Sanidine, and the intermediary member of this series is Anorthoclase. Albite can contain up to ten percent potassium replacing the sodium. If more than ten percent replaces the sodium, the mineral is no longer Albite, but Anorthoclase.

Albite is a very common mineral, and is an important rock-forming mineral. It takes a longer time for Albite to crystallize than the other feldspars. This enables Albite to form in large and well crystallized examples. Albite is also a common accessory mineral to many rare and unusual minerals. The iridescent variety Peristerite sometimes exhibits an adularescent sheen. This produces an unusual form of Moonstone.

Albite sometimes forms in association with the feldspar mineral Microcline in alternating patterns, and forms a feldspar rock known as Perthite.
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Almandine is the most common member of the Garnet group. It is also a popular gemston...  More

Almandine is the most common member of the Garnet group. It is also a popular gemstone and the most widely used Garnet in the gem trade. More gemstones are faceted from Almandine than any other type of Garnet. Only a small amount of Almandine crystals are transparent and light enough for gemstone use; most of the Almandine found is rough and opaque and not gem quality. Some Almandine Garnets display asterism when polished as cabochons, and are known as "Star Garnets".

Almandine is often embedded in a mica schists, and forms very nice matrix pieces with perfectly formed symmetrical crystals. The schist matrix often breaks up due to weathering, resulting in the Almandine crystals breaking loose into individual, perfectly formed floater crystals which may be quite large.  Hide

Amethyst is a well known mineral and gemstone. It is the purple variety of the minera...  More

Amethyst is a well known mineral and gemstone. It is the purple variety of the mineral Quartz, and its most valuable and prized variety. Its name derives from the Greek "amethystos", which means "not drunken", as Amethyst in antiquity was thought to ward off drunkenness. The color of some Amethyst specimens from certain localities slowly fade upon prolonged exposure to light. When used as a gemstone, Amethyst is often heat treated to deepen the color, or to transform it into Citrine. Some varieties may also change to a light green color, which is given the trade name "Prasiolite", or "Green Amethyst", as it is more commonly known in the gem trade.

Amethyst is most prevalent as small stubby pyramidal crystals, although several localities such as the Mexican occurrences are well-known for producing elegantly tall prismatic crystals, which are very highly regarded by collectors. Amethyst also forms the internal lining of geodes, some of which can be over 10 feet tall and weighing several tons.

For additional information, see the gemstone section on Amethyst.
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Analcime is a member of the zeolite group , and often occurs together with other zeol...  More

Analcime is a member of the zeolite group, and often occurs together with other zeolites. However, it is closely related in structure to the feldspathoid group, and is occasionally also classified as a feldspathoid together with the similar mineral Leucite. However, most mineralogical sources, including the IMA, classify Analcime only as a zeolite and not as a feldspathoid.

Although the crystal structure of Analcime appears isometric, its is usually off by only a fraction of an angle. This technically removes its classification from the isometric crystal system. Analyses of different Analcime crystals have yielded multiple results in crystal symmetry, with the most prevalent symmetry being tetragonal. However, truly isometric examples have been analyzed and do exist, thus making this mineral complex in its crystal classification. Due to Analcime's appearance as an isometric mineral, and the fact that isometric examples do exist, we have placed this mineral within the isometric group for classification.

The name Analcime is derived from the Greek term "an alkimos", meaning "not strong, in allusion to the weak pyroelectricity exhibited by this mineral.
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