wombat1138 | Google Books PCR

"Walking" text through snippet-view sources for info about "dragon's breath" glass.

Rocks and minerals, Volume 47‎ - Page 208; Peter Zodac, Rocks and Minerals Association - Juvenile Nonfiction - 1972:

Glass opals are sometimes sold. Generally they are orange-red like the Mexican opal, with blue fire. Look at the stone sidewise (into the edge, parallel to the base) for a thin cement line where two pieces of glass were joined. Mexican red opals (if they have fire at all) display golden yellow or lime green colors, never blue. ...

Jewelers' circular, Volume 123‎ - Page 151; Antiques & Collectibles - 1952:

There has never been a satisfactory or even convincing imitation of opal, though there are a lot of bad imitations around. The commonest appears to be a double layer of orange glass, with an irregular lower surface of the upper layer reflecting a bluish sheen. Sometimes these ...

The Lapidary journal, Volume 19‎ - Page 1160; Lelande Quick - Antiques & Collectibles - 1965:

The common imitation opals, while they do not strongly resemble genuine opal are often quite attractive. They are very often made by fusing together two pieces of appropriately colored glass, after as impalpable metallic coating has been applied to the surface ...

Also, print link for Guyot samples.