Last Updated: Jul 19, 2016 Views: 30
According to Harold Newman's Illustrated Dictionary of Glass, onyx glass "is a type of glass that is dark coloured, usually brown, with streaking of white and other colors, made by mixing molten glass of various colours." Ancient Roman glassmakers were interested in imitating the effects of natural semi-precious stones, such as agate, chalcedony, onyx, lapis lazuli, and jasper. They mixed several colors together in order to achieve these effects.
I am enclosing a bibliography on ancient glassmaking. Many of these materials will be available to you locally, either at your local library or through interlibrary loan.