438 SYSTEMATIC DESCRIPTION OF PRECIOUS STONES 



LAPIS-LAZULI. 



J.apis-lazuli, or azure stone, also known as oriental lapis-lazuli, is an opaque mineral, 

 usually of a magnificent blue colour. It occurs in nature in extremely fine-grained to 

 compact masses with an uneven fracture. The crystals never exceed the size of a pea or 

 bean, and are extremely rare ; they have the form of the rhombic dodecahedron, and 

 belong to the cubic system. 



The beauty of this stone depends entirely upon its colour. In the finest and best 

 specimens it is a dark azure-blue shading off to blackish-blue. Plate XX., Fig. 1, represents 

 a stone of a fine azure-blue with a slight tinge of black. Deep blue specimens only are 

 used as gems, and their colour is far more intense and beautiful than is that of any other 

 opaque blue stone ; it is always deeper than that of turquoise, and on this account the two 

 stones may be readily distinguished. Very pale blue or almost colourless lapis-lazuli is also 

 met with not infrequently ; such specimens might possibly be confused with turquoise were 

 it not that they are so seldom used as gems. As in similar cases stones of a pale colour are 

 described as being " feminine," and those of a deep blue as " masculine." The colour may 

 be distributed with such perfect regularity that the stone is of one uniform tint throughout, 

 but more frequently there are bands and patches which are white or some shade of blue 

 differing from that of the mass of the stone. Moreover, the blue colour is often flecked 

 with yellow, shining, metallic specks of iron-pyrites, which are often supposed to be gold 

 by the uninformed. On the decomposition of the iron-pyrites the yellow specks are 

 replaced by patches of rusty brown, which much disfigure the stone. The precious stone 

 known to the ancients as sapphire was probably the blue lapis-lazuli, and not the blue 

 variety of corundum which now bears that name. 



Lapis-lazuli, showing shades of colour other than the pure blue, is by no means 

 uncommon. At certain localities there frequently occurs a blue variety slightly tinged with 

 green, but stones of a pure green are very rare, and the same is true for violet and reddish- 

 violet stones, which also appear to be confined to certain localities. It is unusual for a 

 stone to exhibit more than one colour, but blue, green, and red or violet are occasionally 

 seen in the same specimen. When reduced to powder the mineral is always of the same 

 tone of colour, but lighter in shade than it was before. 



The colour of lapis-lazuli is not perfectly stable under all conditions ; it is altered, for 

 example, by the action of heat. If a pale blue stone be raised to a dull red-heat it 

 frequently assumes a fine dark blue colour, the specimen which was comparatively worthless 

 before is now fit for use as a gem, and has a considerable value. In other cases the pure 

 pale blue or dark green is transformed into a greenish-blue, which is not much admired ; 

 and if the temperature is raised too high the stone is completely decolourised. When 

 green and violet stones are raised to a dull red-heat they frequently behave like the pale 

 blue stones, and assume a deep blue colour, which in this case also greatly enhances their 

 value. A greenish-blue lapis-lazuli from Chile loses its colour on heating, but regains it 

 during the process of cooling. 



The finely granular structure of lapis-lazuli is apparent even to the naked eye. In 

 the mass the mineral shows no trace of cleavage, and the fracture is sub-conchoidal to 

 uneven. The lustre on a freshly fractured surface is of the vitreous type ; it is usually 



