The mini-Hakkero (ミニ八卦炉, "mini-Eight Trigram Furnace" or "mini-Elemental Furnace") is a magical furnace owned by Marisa Kirisame. As revealed in Curiosities of Lotus Asia, it was originally given to Marisa by Rinnosuke Morichika after she stopped living with her parents.
The mini-Hakkero takes the shape of an octagonal block, made of wood, with the eight trigrams printed in a circle on its front. The internal components were originally made of iron or steel, but replaced by the mythical hihi'irokane metal after it became rusted. Despite its small size (it fits easily in the hand), it is a potent source of heat and magical energy; Rinnosuke describes it as capable of reducing a mountain to ashes. Marisa relies on it heavily for research and combat, most notably to power her Master Spark.
In addition to its normal functions, one of the furnace's corners releases a cool breeze, and the hihi'irokane model gleans negative ions from the air (a function copied from a pseudoscientific device from the Outside World, which nevertheless works inside Gensokyo). Rinnosuke also believes that the mini-Hakkero may function as an amulet for protection and good fortune.
Rinnosuke stated that it is his own invention, which merges the magic in Gensokyo and the Outside World's technology (which he refers to as an outside form of magic that needs no spiritual power). He has also repaired and upgraded it at least once. 
In Mountain of Faith, Marisa has mini-Hakkeros as options.
The original Hakkero appeared in the legend Journey to the West, where it was a furnace in which the monkey king Son Goku/Sun Wukong was imprisoned as punishment after devouring the peach of immortality.