Friday, 22 February 2013

Introduction to Pyrope-Almandine Garnet Gemstone

Crystals of almandine garnet
Silicate of magnesium, iron and aluminum belonging the garnet family. It is more correctly a group of mined consisting of mixtures in variable proportions of theta end-members (the theoretical extremes are rarely ilea found in nature). Mixtures in which magnesium clearly dominates over iron are called pyrope; those in which iron predominates are called almandine.

The name garnet, now applied to the entire family, is originally given to the garnets of the pyrope-almandine series due to their resemblance to red pomegranate seads, (Latin name, and malum granatum)

Crystal system

Usually in isolated, granular crystals, alter in the form of a perfect rhombic decahedron. Thecae, is often reddish brown, but can be a definite red, light red violet red, or deep blackish red. The crystals which often semiopaque can be transparent and limpid highly lustrous faces. They have no cleavage.

Physical properties
The hardness is 7 or 7.5 (the higher figure refers to almandine). The density ranges tor' 3.65-3.87 g/cm3 for pyrope, to 3.95-4.20 g/cm3 in from almandine. Likewise, the refractive index of alts 1.730-1.751 for pyrope gradually increases, parallel the density, to 1.76-1.83 for almandine.

Pyrope is normally found in peridotitic and eclogitic rocks and also in diamond-bearing kimberlitic. Almandine is a characteristic mineral of metamorphic via Due to their resistance to weathering, pyrope and aim dine are often found in alluvial, secondary deposits or arenaceous rocks.

This mineral is very widespread. Countries famous for garnets usable as gems include Czechoslovak! (Bohemia), South Africa, Madagascar, the United Slats Mexico. Brazil, Australia, Burma, Sri Lanka, and India.

Magnesium-rich member of the pyrope-almandine sees The name comes from the Green pyropos, memo "fiery," and is therefore similar to the Latin name carbunculus(small coal or ember), attributed to all red transparent stones. Its red color, sometimes very bright, is due to small quantities of chrome in the crystal structure.

Usually bright red, pyrope can be a much less attractive brick or dark red. It can be perfectly liars parent, but this feature is less visible in dark specimens is either made into fairly convex cabochons, like almandine garnet, or faceted, with an oval or round mixed cut of more rarely, a step cut. The faceted gems have good lusts rather less obvious in cabochons. The most valuable type is of course, the transparent ones with the brightest red color. Pyrope is relatively common, although less so than almandine. Very large stones, up to several hundred carats, have been found; but these are rare and are lea in museums and famous collections.

Distinctive features
Inclusion in garnet Pyrope is singly refractive, but sometimes displays anomalous patches of birefringence and has a luster comparable to that of ruby and spinel. It is distinguished from the former by a lack of pleochroism and the fact that it does not turn bright red in strong light; its can only be distinguished from the latter by measuring its physical characteristics. These are, however, soma* variable, as iron is inevitably present, due toils isomorphic relationship with almandine. It is called pyrope when density is between 3.65 and 3.87 g/cm3 and the retreats index is between 1.730 and 1.751. (Stones with hi" densities and refractive indices are called pyrope-almandine and then almandine.) The hardness is 7 or a RP more.

In the latter half of the nineteenth century most pyrope came from Bohemia, where it is still low today. The main sources nowadays, however, are Soul Africa, Zimbabwe, Tanzania, the United States, Meer Brazil, Argentina, and Australia.

It is of quite low value as secondary gems; probably due to its abundance. The darkest specimens which are the most common are worth very little while even in early times pyrope could be distinguished Inn ruby because of its relative softness, it was more highly valued then than it is today, probably because of its color

Simulants and synthetics
Formerly, when it was mar highly prized, pyrope was imitated by glass, which canto very similar, but does not have the same hardness. It is not produced synthetically.

There is an intermediate group of garnets in the pyrope almandine series. It is a deep pink or pinkish-red cote (known as rhododendron pink) and is called rhodolite, the Greek rhodon, "rose," and lithos, "stone." Rhodolite is a subvariety of pyrope-almandine characterized by its particular color.

Almandine, Namibia.
The pinkish-red color is its main characteristic. The gems have good transparency and are almost always faceted, generally receiving a mixed, roughly oval round cut. As always with transparent garnets the luster is strong.

Distinctive features
The color luster and single reflection typical of garnets paint quite a clear picture. The store is not, of course, pleochroic. The refractive index various from 1.755 to 1.765 and the density from 3.74 to 3.1 g/cm3 a very limited range. Rhodolite is distinguish from corundum of a similar color by its lack of pleochroism and the fact that it does not fluoresce in bright light are from rubellite by its lack of pleochroism, greater luster," absence of the marked birefringence of tourmaline. At is even visible with a lens. Its physical properties have to be measured, however, to distinguish it from spinet.

Rhodolite is found in the United Stars Zimbabwe, Tanzania, and Sri Lanka. It is not common

Of the reddish garnets, it is in greater demand and therefore more valuable than pyrope and almandine.

Simulants and synthetics
It is neither imitated, nor produced synthetically

Most remaining red garnets (usually a deep, violet-red) come under the name of almandine, even when their cor. position is midway between that of pyrope and almandine and similar, in many cases, too that of rhodolite. The reason for this is the similarity in their color and absorption spectrum characteristics. The name almandine comes from carbunculus alabandicus, after the city of Alabandair Asia Minor, where gems were traded at the time of Pliny the Elder (carbunculus, as already explained, means -seat coal" and has been used to refer to red stones in general).

Almandine cabochon
The color is red, but often a deep. Violet-red, It has brilliant luster, but its transparency is frequently marred, even in very clear stones, by excessive depth al color. The cabochon cut is widely used, often being given a strongly convex shape and sometimes a concave base, in an effort to lighten the color by reducing the thickness. Rose cuts have also been used, particularly in the past Nowadays, when the material is quite transparent, faceted cuts are used as well, and sometimes square or rectangular step cuts. Gems of several carats are not uncommon Faceted or even barely rounded pieces of almandine pierced as necklace beads, were very common in the recent past, but are now considered old-fashioned.
Distinctive features
The deep, almost violet-red is fairly typical, and has given rise to the expression -garnet red.' It is not enlivened, as are dark rubies, even by strong light and its single refraction and lack of visible pleochroism should normally distinguish it from similarly colored rubellite. If fairly transparent faceted stones are viewed from above, some of the facets often look back on the inside (this is known as the "garnet effect"). Almandine has a luster comparable to that of corundum. It is not easily distinguished from spinel, except by examining the physical properties, which can vary quite considerably, according to the composition: the density is between 3.95 and 4.20 g/cm3, or thereabouts; the refractive index varies from 1.76 to 1.83 (the density increases parallel to the index), It has a hardness of about 6-7.5.

Crystals of spessartine garnet Italy (Island of Elba)Almandine is obtained in large quantities from Sri Lanka and India, where it is also cut: other sources are Burma, Brazil, the United States, Madagascar, Tanzania, and Australia.

This depends on brightness of color and two from cracks and inclusions, but is always quite low. Almandine was extremely popular in the nineteenth century but the name "garnet" is now automatically as   with cheap stones.

Simulants and synthetics
Almandine has been imitated by glass, which can look very similar. It has apparently been produced synthetically, but not on a commit basis.

Writer – Curzio Cipriani and Alessandro Borelli

No comments:

Post a Comment