Miraculouses are magical jewels. Worn as jewelry, they have the ability to change their wearers, with the help of kwamis, into animal-themed, super-powered beings.
According to Nooroo, Miraculouses, magic jewels bestowing amazing power, were created many centuries ago. Meant for good purposes, the Miraculouses have been used by heroes throughout history. However, in the wrong hands, Miraculouses may be used for evil.
A Miraculous, when worn and inhabited by a kwami, transforms the wearer into a costumed appearance based on its animal theme. It also grants the wearer enhanced abilities, a weapon, and a unique superpower. Some of a Miraculous' powers follow a theme, like the Ladybug Miraculous having the power of creation, but the kinds of powers a Miraculous has aren't limited to the theme. The transformation lasts until the superpower is used, leaving the wearer about five minutes until they revert back to normal.
A brooch that turns its wearer, with help from Duusu, into a peacock hero. Currently, it is kept by Gabriel Agreste inside a safe behind a large painting in his mansion.
The Ladybug Miraculous and Cat Miraculous are set opposite of each other in a yin-yang section in Master Fu's chest. The Cat Miraculous is placed in yang, and the Ladybug Miraculous is placed in yin.
They are considered the most powerful out of all the Miraculouses.
A person can wear more than one Miraculous and use them both at the same time.
Legend says that if the Cat Miraculous and the Ladybug Miraculous are worn together, the wearer gains absolute power.
There are only seven known Miraculouses so far, which are stored on the top level of Master Fu's chest, but Thomas Astruc explained that it's a bit more complicated than that, with more information being discovered about them throughout the three seasons of the series.
In the animatic of the theme song, the chest box has many Miraculouses that are not used in the final version, including what appears to be an Eagle Miraculous, Snake Miraculous, Bear Miraculous, Lion Miraculous, Chicken Miraculous, and Bull Miraculous.
The Snake Miraculous, in particular, is a bracelet depicting an Ouroboros, a serpent eating its own tail. It is a symbol of self-reflexivity, cyclicality, and eternal return.
Kwamis go back into their Miraculous if the holder disposes of it intentionally.
However, kwamis do not return to their Miraculous in dormant form if the holder involuntarily disposes of it, as seen in "Lady Wifi" when Cat Noir loses his ring.
According to what Master Fu says in "Princess Fragrance", the Miraculouses might be connected to ancient China.
The official Miraculous Ladybug Tumblr blog noted that the TV series, comic books, and web series will show the history of the Miraculouses.
In "Ladybug & Cat Noir (Origins - Part 1)", when Wayzz asks whether the new Ladybug and Cat Noir will be up to the task, Master Fu states that "[he] only got it wrong once," probably referring to him assigning a Miraculous to a wrong person in the past.
While each Miraculous has a theme, like the Ladybug Miraculous having the power of creation and the Cat Miraculous having the power of destruction, the powers a Miraculous has are not limited to the Miraculous' themes.