Crystals, Fine Crystals

Peridot

Mogok in Burma is known to produce the best quality for peridot with its two mines near the Barnardmyo villages: Pyaung Gaung and Zalat Thaung. Pakistan is also renowned for the exceptional quality of its stones but most of the peridots in the market nowadays are coming from San Carlos in Arizona, USA and China. Chinese and US peridots are plentiful but their color is much more yellowish green than their Burmese or Pakistani counterparts. Egypt is an important historic source.

The Myanmar, Pakistani and Egyptian gems are rarer and of better quality and thus quite valuable, approaching the per carat values of top gemstones. Peridot is the gem variety of peridotite, which is a very common mineral in the earth's crust. But Peridots are found in ultra basic rocks mostly in hard rock mines. Lilipad is a typical diagnostic inclusio found in many peridots: It shows a disk-like liquid surrounding a dark octahedral chromite crystal.

The Romans referred to "Peridot" as the "Evening Emerald" because the color did not darken at night and was visible under a lamplight. Some traders still call it the "Night Emerald" but this name should not be used now as peridot is less valuable than emerald. The most valuable color for peridot is pure green and so any yellow present decreases the global stone value.

The qualities of the peridot color also increase with the size of the stone and small size, top-color peridot is nearly impossible to find. As most Burmese peridot mines in Mogok are government owned, the Burmese government gem auctions in Yangon attract many peridot foreign dealers. A few fine peridots are found in the limitated Burmese private market.

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Phenakite

The name phenakite, or the alternate spelling, phenacite, is from a Greek word meaning deceiver, an allusion to its deceptively similar look to quartz.

Phenakite is a rare beryllium(Be2SiO4) mineral, but it is found so frequently with precious gemstones that its availablity is not in proportion to its rarity. Basically, Phenacite is found in high-temperature pegmatite veins and in mica-schists associated with quartz, chrysoberyl, apatite and topaz. It has long been known from the emerald and chrysoberyl mine.

Phenakite is one of the few silicate minerals that have a trigonal symmetry. This symmetry is far more common among carbonates than among silicates.

Phenakite shares the same symmetry with the emerald green silicate dioptase and the fluorescent and closely related willemite. Occasionally,

Phenakite is used as a gemstone. Phenacite occurs as isolated crystal which are rhombohedral with parallel-faced hemihedrism, and are either lenticular or prismatic in habit: the lenticular habit is determined by the development of faces of several obtuse rhombohedra and the absence of prism faces. There is no cleavage, and the fracture is conchoidal. The hardness is high, being 7.5 - 8; the specific gravity is 2.96. The crystals are sometimes perfectly colorless and transparent, but more often they are greyish or yellowish and only translucent; occasionally they are pale rose-red. In general appearance the mineral is not unlike quartz, for which indeed it has been mistaken.Fine crystals of phenakite can be perfectly clear and with good hardness, rarity and lack of good cleavage, although somewhat lacking in color and fire, make good choices for gemstones. For gem purposes the stone is cut in the brilliant form, of which there are two fine examples, weighing 34 and 43 carats (6.8 and 8.6 g), in the British Museum. The indices of refraction are higher than those of quartz, beryl or topaz; a faceted phenacite is consequently rather brilliant and may sometimes be mistaken for diamond.

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A crystal or crystalline solid is a solid material whose constituent atoms, molecules, or ions are arranged in an orderly repeating pattern extending in all three spatial dimensions. The scientific study of crystals and crystal formation is crystallography. The process of crystal formation via mechanisms of crystal growth is called crystallization or solidification.

The word crystal is derived from the ancient Greek word ?????????? (krustallos), which had the same meaning, but according to the ancient understanding of crystal. At root it means anything congealed by freezing, such as ice. The word once referred particularly to quartz, or "rock crystal".

Most metals encountered in everyday life are polycrystals. Crystals are often symmetrically intergrown to form crystal twins.

Crystal structure

The process of forming a crystalline structure from a fluid or from materials dissolved in the fluid is often referred to as crystallization. In the ancient example referenced by the root meaning of the word crystal, water being cooled undergoes a phase change from liquid to solid beginning with small ice crystals that grow until they fuse, forming a polycrystalline structure. The physical properties of the ice depend on the size and arrangement of the individual crystals, or grains, and the same may be said of metals solidifying from a molten state.

Which crystal structure the fluid will form depends on the chemistry of the fluid, the conditions under which it is being solidified, and also on the ambient pressure. While the cooling process usually results in the generation of a crystalline material, under certain conditions, the fluid may be frozen in a noncrystalline state. In most cases, this involves cooling the fluid so rapidly that atoms cannot travel to their lattice sites before they lose mobility. A noncrystalline material, which has no long-range order, is called an amorphous, vitreous, or glassy material. It is also often referred to as an amorphous solid, although there are distinct differences between solids and glasses: most notably, the process of forming a glass does not release the latent heat of fusion.

Crystalline structures occur in all classes of materials, with all types of chemical bonds. Almost all metal exists in a polycrystalline state; amorphous or single-crystal metals must be produced synthetically, often with great difficulty. Ionically bonded crystals can form upon solidification of salts, either from a molten fluid or when it condenses from a solution. Covalently bonded crystals are also very common, notable examples being diamond, silica, and graphite. Polymer materials generally will form crystalline regions, but the lengths of the molecules usually prevent complete crystallization. Weak Van der Waals forces can also play a role in a crystal structure; for example, this type of bonding loosely holds together the hexagonal-patterned sheets in graphite.

Most crystalline materials have a variety of crystallographic defects. The types and structures of these defects can have a profound effect on the properties of the materials.