The mini-Hakkero in Silent Sinner in Blue.
The mini-Hakkero (ミニ八卦炉, "mini-Eight Trigram Furnace" or "mini-Elemental Furnace") is a magical device 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 a hand-sized octagonal block of wood, with the eight Taoist trigrams inscribed in a circle around its center. The internal components were originally made of iron or steel, but replaced by the mythical hihi'irokane metal after rusting. It is capable of generating immense 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.
Marisa about to fire a Master Spark using the mini-Hakkero in Strange and Bright Nature Deity.
The mini-Hakkero in Curiosities of Lotus Asia.
Depiction of mini-Hakkero in one of the Ability Cards seen in Unconnected Marketeers.