Fused quartz or silica fibres, of extreme tenuity, but of relatively very great strength, are employed in many scientific instruments, where their extreme lightness and perfect elasticity and freedom from what is known as “elastic fatigue” renders them of very great value. These fibres are not drawn from a mass of molten silica, as is done with glass, but are produced by attaching a nail or bolt to a small bead of fused silica produced by the aid of an oxygen-fed blowpipe; the nail or bolt is then suddenly shot away down a long passage or similar space by means of a cross-bow, drawing a very fine fibre of silica with it; the most difficult part of this operation, however, consists in finding and handling the fibres thus produced. _Artificial Gems._--The fact that pieces of suitably-coloured glass can be made to show a superficial, but sometimes more or less deceptive, resemblance to precious stones, has led to the manufacture of imitation jewels of all descriptions. The glass used for this purpose is usually a very dense flint-glass whose high refractive index facilitates the imitation which is aimed at. The external shapes of gems are, of course, readily imitated by cutting and grinding the glass, while the requisite colours are attainable by means of the colouring materials described in Chapter XI. To a casual observer the difference in sparkle and brilliance which arises from the difference between the refractive index of the heavy flint-glass (about 1·8) and that of minerals (which ranges from 1·7 to 2·2) is not readily apparent, but closer examination will at once reveal the difference. The determination of the optical constants by means of a refractometer would at once reveal the true character of the imitation, but an even readier test is that of hardness. The dense flint-glass is naturally soft, and is readily scratched by most of the harder minerals, while the precious stones, more particularly garnets, rubies and diamonds, are very hard. If an attempt is made to scratch an ordinary sheet of window-glass, it will be found that most real precious stones will do so readily, while flint-glass imitations will fail to make more than a slight mark, which is more smear than scratch. The test by determining the specific gravity is also obviously applicable, since the flint-glass will readily betray its presence by its high density (over 4).