Thereis another possibility; it was used in a fictional novel by Anne Maccaffrey.
"Firestone
Firestone is a phosphine bearing rock found on Pern. When chewed by the Pernese dragons, they are able to produce phosphorus gases, which then ignite and are used, primarily, for burning Thread out of the air. It exists in two varieties. One variant is highly unstable in nature, often prone to releasing their deadly and potentially explosive gases on contact with even the slightest bit of moisture. This type of firestone is extremely dangerous to mine, store and use. The other variant is relatively stable, and require the rocks to be somewhat "digested" in the dragons second stomach's acid before the phosphine gases are released. This was the variety initially discovered on the beaches of the Southern Continent in Dragonsdawn and used in all Pern novels following Dragon's Fire in Pern's chronology.
Firestone is believed to render female dragons sterile, thus requiring queen dragons to use flamethrowers when fighting Thread. This side effect would be beneficial, however, with regard to green dragons (which are also female)—being rather promiscuous, green dragons would cause Pern to quickly become overpopulated with small dragons unsuitable for Thread-fighting duty were it not for the sterility believed to be caused by chewing firestone. This traditional theory is debunked for the reader's benefit in the novel Dragonsdawn, where it is noted that green dragons are sterile by design, as is the queens' inability to properly utilize firestone. Over time, this knowledge was lost, and the myth above became widely accepted."
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The past is a foreign country: they do things differently there.