The
cornelian has sometimes the colour of polished horn; there is one
variety that resembles the hyacinth, and another, of vermilion red,
somewhat analogous to the ruby. Its colouring is due to the oxide of
iron, and in certain varieties to organic matter, clearly discernible
by analysis.
Heliotrope.—A
translucid agate of a lively leek-green colour, spotted with red. The
ancients used it, as Pliny tells us, for looking at eclipses of the
sun, as we use smoked or stained glass; and it was said also to change
the colours of the sun's rays into blood-red, when it was plunged into
a vase of water. Hence the name heliotrope, from the Greek helios, the sun; and trepo, to turn.
SECOND VARIETY.
Onyx.—The onyx is the most celebrated variety of all the variously tinted agates.
Originally the name onyx was given to agates which had the appearance of a nail (Greek, onyx) where
it joins the flesh; but it is now used for stones which exhibit marked
contrasts of colour in bands, as black and white, or black and
whitish-gray.
When
an onyx unites in a desirable degree these conditions, it constitutes a
stone of value, on account of the resources offered by it to the
engraver, through the contrast of colours.
