Silicate of sodium and aluminium, containing sulphur. It is a feldspathoid of the sodalite group.
Crystal system Cubic.
Appearance lt 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 haiiyne. Calcite is also frequently encountered in the form of bright patches or light. even whitish veinings. It is frequently found associated with pyrite.
Physical properties it has a hardness of 5-5.5. The density is about 2.38-2.45 g/cm“, but may be higher in the aggregates used as an ornamental material, due to an abundance ct foreign minerals. The refractive index is about 1.50.
Genesis It is a contact metamorphosed, metasomatically altered limestone.
Occurrence it is mainly found in Afghanistan and Chile. Other sources are the Soviet Union (Siberia), Burma, Aﬂgola, Canada (Labrador), and the United States (California). It is also found in Italy in limestone blocks ejected by Vesuvius.
The name of the gem is derived, through the medieval Latin lapis lazulus, 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 sapphirus 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: lazu_rite and variable quantities of calcite, pyrite, and other teldspathoids of the sodalite group, such as hauyne and nosean.
Appearance it has a uniform, massive or sometimes granular appearance, with fairly distinct crystals. it is semiopaque or opaque. with a surface that can take a good polish-—lil1.e iades, 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 t.he 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 The particular, very attractive color and speckling with minute crystals of pyrite give lapis lazulian unmistakable appearance. As for the physicat 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 ofthe mineral lazurite. It is normally between 2.? and 2.9 g/cma, but can be as much as 3.0 g/cma. On contact with a minute drop of hydrochloric acid, lapis lazuli immediately gives oﬂ an odor of hydrogen sulphide (like the smell of rotten eggs).
Occurrence The best quality lapis lazuli comes from Afghanistan, where it has been mined since remote antiquity. The ancient Egyptians probably obtained their supplies from there. It is also found in Chile, but usually with numerous light patches and veins. Much smaller quantities come from the Soviet Union (Siberia), Burma, Pakistan, Angola, the United States. and Canada.
Value It is one of the most valuable semiopaque ornamental materials, worth about the same as good quality turquoise and the better iades (excluding imperial jade)-
When it contains light veins of other minerals, the value diminishes. but not excessively, as the effect is still very
Sirnulante and synthetics It was and is much imitated by glass. sometimes containing minute specks of metal To simulate pyrite, by stained chalcedony, and by a deep blue sintered aggregate of minute grains of synthetic Spinel (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 ot 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 or ground tragrnents. 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.