placed
a backing of red, gold, copper, or fiery -coloured tinsel, whilst the
glass itself, at the back, is painted very thinly with a paint composed
of well washed and dried fish-scales, reduced to an impalpable powder,
mixed with a little pure, refined mastic, or other colourless varnish.
This gives a good imitation of phosphorescence, as well as a slight
pearliness, whilst the tinsel, seen through the paint and the curious
milkiness of the glass, gives good " fire." A knowledge of the colours
natural to precious stones and to jewels generally is of great service
in their rough classification for testing, even though some stones are
found in a variety of colours. An alphabetical list of the most useful
is here appended, together with their average specific gravities and
hardness. (See also Chapter VII. on " Hardness," and Chapter VIII. on
"Specific Gravity.")
White or Colourless Stones.
