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Oligoclase belongs to the Plagioclase Feldspar group, an isomorphous solid solution s...  More

Oligoclase belongs to the Plagioclase Feldspar group, an isomorphous solid solution series. Albite is one member, containing sodium and no calcium. The other end member, Anorthite, contains calcium and no sodium. Oligoclase is an intermediary member of this series. Oligoclase is considered by some authorities as a variety of Albite rather than a separate mineral. The acclaimed Dana's System of Mineralogy lists Oligoclase as an individual mineral, whereas the IMA does not recognize it as individual mineral species, but rather a calcium-rich variety of Albite.
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Olivine is one of the most common minerals in the earth, and is a major rock forming ...  More

Olivine is one of the most common minerals in the earth, and is a major rock forming mineral. Despite this, good specimens and large crystals are uncommon and sought after. Only few localities yield large examples of this mineral, although small and microscopic grains are found worldwide. Olivine is also found in meteorites, and large grains have been reported in many of them.

Olivine is not scientifically classified by the IMA as an individual mineral species, but is rather recognized as a mineral group with the Forsterite and Fayalite end members. Fayalite and Forsterite create a solid solution series, and most specimens identified as Olivine fall somewhere in between this series, almost always leaning more towards Forsterite with a greater content of magnesium. Pure Forsterite is uncommon, and pure Fayalite is very rare.
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div Olmiite is a relatively new mineral, only being officially named as a new species...  More

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The rich play of color in some Opals gives them unsurpassed splendor and mystique. Fo...  More

The rich play of color in some Opals gives them unsurpassed splendor and mystique. For this reason, Opal is one of the most fascinating and fabled of gemstones.

Opal, being amorphous, is not truly a mineral but a mineraloid. One of the scientifically accepted standards defining a mineral is that a mineral must have a crystal structure, which opal lacks. Despite this, virtually all scientific references, including the acclaimed Dana's System of Mineralogy, categorize Opal together with the true minerals.

Many theories attempted to explain the cause of the play of color in Opal. In the 1960's, the reason of the color play was discovered with the aid of the electron microscope, where it was determined that Opal is composed of tiny silica spheres that can be arranged in an orderly pattern. This diffracts the light entering the stone into the spectral colors. A light wave diffracted through the Opal causes a color sheen or scintillation in the stone. The density and pattern of the aligned silica spheres are responsible for the different colors refracted in the Opal. Common Opal lacks this effect, since its spheres are disordered or too compact to permit the light from refracting.

A condition called crazing affects certain Opals, causing them to form internal and external cracks. Crazing is a particularly interesting phenomenon, since it lacks consistency and is unpredictable. Although it can occur at random, it usually strikes when an Opal removed from damp conditions is allowed to dry too quickly, or when an opal is exposed to sudden intense light (or a combination of these factors). Crazing may also take place when an opal is subject to vibration, as during the cutting and polishing of a specimen. The severity of the crazing and the time it takes to "craze" varies among specimens. The origin of the specimen is often a determining factor to its resistance to crazing. A very gradual drying process over months or even years can in some cases effectively stabilize the stone and allow it to be cut and polished with a substantially reduced risk of crazing.

Uncut Opals are often stored in water; this reduces the chance of crazing. Once a specimen is taken out of the water its susceptibility increases. Opal stored in water should not be taken out of the water for more than several minutes at a time. Cutting or polishing Opals, especially Opals from localities notorious for crazing, is a risky process; it is a matter of chance if the Opals will craze or not. To further protect Opals from crazing, they should not be washed with chemicals or detergents and should not be subject to sudden changes in temperature or lighting.

For additional information, see the gemstone section on Opal.
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Orpiment is known for its strikingly bright yellow color, though it can also be bright orange or lustrous brown. The name Orpiment is derived from the Latin Auripigmentum, meaning gold pigment, in reference to its color and historical use as a golden-yellow pigment. Orpiment is often associated with bright red Realgar, and may form in strikingly colored and unique mineral combinations.

Orpiment is a photosensitive mineral and will eventually dull and develop a white powdery film upon prolonged exposure to light. Due to the instability of Orpiment, specimens should be stored enclosed and covered to prevent their exposure to light. Occasional exposure to look at a specimen will not cause damage; only prolonged or repeated exposure will cause deterioration.

Orpiment contains a significant amount of poisonous arsenic, and is itself somewhat toxic. Washing hands is recommended after handling Orpiment specimens, especially if powdery.
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Orthoclase is one of the most common minerals, and occurs in numerous mineral environ...  More

Orthoclase is one of the most common minerals, and occurs in numerous mineral environments. It is polymorphous with the minerals Microcline and Sanidine. These three minerals form the Potassium Feldspar group. They are almost identical in physical properties, and it is sometimes impossible to distinguish one another without x-ray analysis. The only difference between them is their crystal structure. Microcline crystallizes in the triclinic system, and Orthoclase and Sanidine crystallize in the monoclinic system. Sanidine forms at high temperatures and has a disordered monoclinic symmetry, whereas Orthoclase forms at low temperatures and cools slowly, forming more ordered monoclinic crystals.

In some mineral reference guides, Microcline and Sanidine are wrongly categorized as variety of Orthoclase. Since it is so difficult to distinguish between Orthoclase, Sanidine, and Microcline, they may be simply called "Potassium Feldspar".
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Otavite is named after the famous mineral deposit of Tsumeb, in Otavi Province, Namib...  More

Otavite is named after the famous mineral deposit of Tsumeb, in Otavi Province, Namibia. Otavite belongs to the calcite group of minerals, a group of related carbonates that are isomorphous with one another. They are similar in many physical properties, and may partially or fully replace one another, forming a solid solution series. All members of the calcite group crystallize in the trigonal system, and have perfect rhombohedral cleavage.  Hide