Thursday, 8 August 2013

Lazurite (Lapis lazuli)

Lapis lazuli Silicate of sodium and aluminium, containing sulphur. It is a feldspathoid of the sodalite group.

Crystal system

Cubic.

Appearance

It occurs as aggregates of crystals, which are sometimes microscopic, sometimes about a millimeter, or even several millimeters in size. The aggregates are, however, thick, their color tending toward dull violet, due to the presence of sometimes numerous minerals of the Same group such as sodalite, nosean, and hauyne. Calcite is also frequently encountered in the form of bright patches or light, even whitish veinings. It is frequently found assn. ciatral with pyrite.

Physical properties

It two; a hardness; of 5-5.5 the density is about 2,3-2.4b g /cm', but muy by higher In the aggregates used as an ornamental materiel, due to an abundance of foreign minerals. The refractive index at about 1.50

Genesis

It Is a contact metamorphosed, met somatically altered limestone.

Occurrence

It is mainly found in Afghanistan and Chile. Other sources are the Soviet Union (Siberia), Burma, Angola, Canada (Labrador), and the United States (California). Ills also found in Italy in limestone blocks ejected by Vesuvius.

Lapis lazuli

Lapis lazuli penThe name of the gem is derived, through the medieval Latin lapis lazuli's, from the Arabic word lazward, from which the word azure comes; but according to the description of Pliny the Elder, the ancient Romans called it sapphirus. The name sapplirrus was, of course, subsequently applied to the blue variety of corundum. Scientifically speaking lapis lazuli is a "rock," because it consists of an association of minerals: !azurite and variable quantities of calcite, pyrite, and other feldspathoids of the sodalite group, such as hanyne and nosean.

Appearance

It has a uniform, massive or sometimes granular appearance, with fairly distinct crystals. It is semi-opaque or opaque, with a surface that can take a good polish like jades, for example. It is a strong but lively blue, sometimes with a hint of violet. It often contains grayish or off-white patches or veins, consisting of distinct, interwoven crystals which are minutely fringed at the edge of the patches, interpenetrated by and interwoven with the minute crystals of blue. The presence of white patches reduces the gem's value. The most highly prized varieties are those which are uniformly colored, preferably without a violet tinge. It often contains minute, scattered crystals of pyrite, which do not detract from its value. It is made into spherical or curved beads and even faceted, polyhedral ones, in which the flat facets can take a very good polish. It is also fashioned into carved gems, boxes, mosaics, small ornaments, vases, and figurines, the largest of which may be tens of centimeters in size. At one time, it was much used for sealstones. The Egyptians used it for their cylindrical seals.

Distinctive features

Lapis lazuli Art
The particular, very attractive color and speckling with minute crystals of pyrite give lapis lazuli an unmistakable appearance. As for the physical properties, the density of gem-quality material is very variable due to the presence of pyrite and other foreign minerals, but in any case, it is much higher than that of the mineral lazurite. It is normally between 2.7 and 2.9 g/cm3, but can be as much as 3.0 g/cm3. On contact with a minute drop of hydrochloric acid, lapis lazuli immediately gives off an odor of hydrogen sulphide (like the smell of rotten eggs).

Occurrence

The best quality lapis lazuli comes from Afghanistan, whore it has hone mined airier; remote Belieudy. The ancient Egyptians probably obtained their sup-plies from there. It is also found in Chile, bidlinually with numerous light patches and voins. Much arnallei tier, some from the Soviet Union (Siberia), Burma, Pakistan, Angola the United States, and Canada.

Value

It is one of the most valuable somiepaque ornamental materials, worth about the same as good quality turquoise and the better jades (excluding imperial jade). When it contains light veins of other minerals, the value diminishes, but not excessively, as the effect Is still very pleasing.

Simulants and synthetics

Lapis lazuli NecklaceIt was and is much imitated, by glass, minute specks of metal 10 simulate pyrite, by stained chalcedony, and by a deep him motored aogregato of minute grains of synihritis spine (plus the usual metal fragments to simulate pyrite). A product has recently appeared on the market which is extremely homogeneous, very deep blue with a violet tinge and scattered with minute fragments of pyrite. This is called synthetic lapis lazuli, although it does not correspond exactly with the natural stone in chemical composition. Furthermore, as the pyrite consists of ground fragments, it never displays the characteristic crystal form. The white patches in low quality lapis lazuli are sometimes colored blue and this practice is not always easy to detect Natural stones are sometimes impregnated with paraffin to improve surface polish and heighten the color. This is the same procedure often used with poor quality turquoise, but the effects are not so far-reaching, perhaps because lapis lazuli is much less porous.

Writer - Curzio Cipriani & Alessandro Borelli

1 comment:

  1. Very useful and Structured Information !
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    Thanks again for this blog !

    ReplyDelete