In the novel The Mystery of the Blue Train, Heart of Fire is a famous ruby. During the events of the novel, Rufus Van Aldin purchases it, along with several other rubies, to give to his daughter, Ruth Kettering.
According to Van Aldin, Heart of Fire is perfect, and there is "not a flaw in it". He paid four or five hundred thousand dollars for the rubies, including Heart of Fire. Aside from their monetary value, Van Aldin also says that the rubies have historical interest. He mentions that Catherine of Russia wore them.
Heart of Fire is said to have "left behind it a trail of tragedy and violence". As Van Aldin watches Ruth holding the rubies to her breast, he thinks of the series of women who have worn them, and the accompanying heartache, despair, and jealousy.
Ruth is later murdered, and the rubies, including Heart of Fire, are stolen. It is implied that by the end of the novel, Heart of Fire has been purchased by a Greek ex-Minister, and given to Mirelle. By this time, it is not the centre stone in a necklace, but a single stone the size of a Pigeons egg, hung on a platinum chain around Mirelle's neck.
In the Agatha Christie's Poirot episode The Mystery of the Blue Train, Van Aldin purchases Heart of Fire as a single stone. In this adaptation, Heart of Fire is not resold after being stolen, and the murderer is revealed to have it with them during the denouement. The murderer attempts to take someone hostage, holding the sharp point of the ruby against their throat. However, after Poirot tells the murderer that they are a great jewel thief, but not a mere lunatic, they release the hostage. The murderer hands the ruby to the hostage, before committing suicide. It is not known what happens to the ruby after that.