496 SYSTEMATIC DESCRIPTION OF PRECIOUS STONES 



B, COMPACT QUARTZ, 



HORNSTONE. 



HornstDne is a very fine-grained to perfectly compact quartz, consisting of an aggregate 

 of microscopically small grains of the mineral. The sharp edges of broken fragments are 

 slightly translucent, and the stone is further characterised by its splintery fracture. These 

 two characters serve to distinguish hornstone from jasper, the latter having a smooth 

 fracture, and being perfectly opaque, even in the thinnest of splinters. Hornstone is 

 usually of a dingy grey, brown, or yellow colour, so that it resembles horn in colour as well 

 as in being slightly translucent, hence its name, which is an old mining term and not used 

 by lapidaries at all. As used by mineralogists, the term includes silicified wood and 

 chrysoprase, the two varieties of hornstone which are cut as ornamental stones, as well as 

 several other stones which are not used for this purpose, and which, therefore, need not be 

 mentioned here. 



Most hornstones, such as occur abundantly in many metalliferous veins and as nodules 

 in limestone, clay, &c., do not possess the characters necessary for gems, but there are one 

 or two varieties which are more attractive in colour, and which are therefore sometimes cut. 

 One of these, known as chrysoprase, is green in colour ; and another, wOod-stone, shows the 

 structure of wood, and is in fact petrified wood. The surface of wood-stone is sometimes 

 prettily marked, and the substance is often fashioned into small ornamental objects of 

 various kinds. It is similar in many respects to wood-opal, already described, but differs 

 from it in the fact that the petrifying material is quartz instead of opal. 



Wood-stone (fossilised or silicified wood). — This stone has been formed by the 

 impregnation with quartz of the woody substance of plants of former geological ages, and 

 its gradual replacement by the same mineral. The quartz, thus deposited, possesses 

 characters which, on the whole, resemble most nearly those of hornstone. Not only is the 

 structure of the wood well preserved, but the identity of the original object can be easily 

 recognised by reason of the perfect preservation of the characteristic external form of its 

 trunk and branches. When the material is cut, either longitudinally or transversely, and 

 polished, the woody structure becomes still more apparent. A longitudinal section along 

 the axis of the trunk or branch displays the cells and vessels, alternating bundles of which 

 are frequently coloured. Transverse sections show these cells and vessels in section, and so 

 perfectly are details of structure preserved that when thin sections are examined under the 

 microscope it is often possible even to determine the nature of the original plant ; many 

 species of fossil palms, pines, &c. have, indeed, been so determined. 



In wood-stone the walls of the vessels are usually of a dusky brown colour, while the 

 material which fills up the cavities of the vessels and the spaces between neighbouring 

 bundles is usually much paler in colour. This only serves to show up still moi-e clearly the 

 woody structure, which, with the high lustre acquired after polishing, renders wood-stone 

 very effective for ornamental purposes. The markings of a cross-section often recall the 

 speckled plumage of the starling, and such specimens have been referred to as starling-stone. 

 The material is occasionally cut for mounting as a gem, but more frequently, though now 

 less than formerly, it is fashioned into small semi-ornamental objects. Wood-stone is the 

 material which the ancient Babylonians used for some of their cylinder-seals. 



The rough material is widely distributed, and the value of worked articles is little 

 higher than the cost of the labour employed in their manufacture. In Germany wood-stone 



