22 GEMS IN GENERAL. 
The ancients engraved upon several kinds of gems; 
but they appear to have been ignorant of the art of 
cutting the diamond, the ruby, and the sapphire, which 
were too hard for them to operate upon. The emerald 
and the noble opal (102) were too highly esteemed as 
precious stones to have often found their way into the 
hands of engravers. It has been asserted that the an- 
cients did not use the topaz for engraving ; but there 
is extant a beautiful intaglio, representing an Indian 
Bacchus, which is said to be a topaz. The garnet was 
often engraved upon : and there are many master-pieces 
of the art in calcedony (91) and carnelian (93.) Onyx 
and sardonyx (92) were employed for that species of 
engraving in relief called cameos ; and, in many in- 
stances, it is pleasing to observe with what dexterity 
the ancient artists availed themselves of the different 
colours in the alternate zones to express the different 
parts and shades of their figures. 
Most of the gems may be imitated by artificial pre- 
parations of glass, coloured by different metallic sub- 
stances ; and it is not easy, by mere inspection, to dis- 
tinguish the better kinds of factitious stones from real 
gems. They are, however, discoverable by a deficiency 
of lustre, and being so soft as, even in the most perfect 
kinds, to yield to the point of a steel instrument. 
The cutting and polishing of gems is the work of the 
lapidary, and is in general thus performed : The shape 
most proper to be given to any particular gem being 
determined on, the stone is cemented to the end of a 
stick, and the different facets are formed by a mill 
contrived for the purpose. This mill is a plate of cop- 
per, or an alloy of lead and tin, to which an horizontal 
motion is given by very simple machinery, and the sur- 
face of which is charged either with diamond powder 
and oil, or with fine emery and water. A thick peg of 
wood called a guage, pierced with small holes in all 
directions, is set upright on the lapidary's bench, close 
to the mill, 'and the process of shaping the facets thus 
takes place. The stone is placed on the surface of the 
