The Regent Diamond is a 141 carat diamond originating from India. It received its name when it was purchased by French Regent Philippe II.
The Queen's Necklace originates from the infamous affair of the Diamond Necklace which further tarnished Queen Marie Antoinette's reputation, leading to the French Revolution and the queen's execution.
The Stone of Scone was used in the coronation of the monarchs of Scotland, and later was put to the same use by the monarchs of England and the United Kingdom.
The Koh-i-Noor is the world's largest natural colorless diamond mined from India. Its uncut form is said to weigh 186 carats. Within the British royal family, the diamond was rumored of bringing bad luck to any man who wears it.
The Orlov is an Indian diamond which has never been officially weighed but estimated to be around 190 carats. It sets in the sceptre named after the Russian Count Grigory Grigorievich Orlov by Empress Catherine the Great.
The Talisman of Charlemagne is an amulet with a True Cross and the supposed hair of Mary between the two large sapphires. It once belonged to the King of the Franks Charles I.
The Hope Diamond is a grayish-blue diamond said to bring curses to the owner.
The Crystal Skulls are pre-Columbian Mesoamerican artifacts. When all 13 skulls are closely gathered it is said to reveal the mysteries of the universe.
Treasure Demons cannot be equipped for combat but can be used in fusion. Fusion with Treasure Demons is very similar to Element race demons in the traditional Shin Megami Tensei series in that a rare Persona will change another ingredient Persona in fusion into another Persona of the same Arcana either rank up or down by 1 or 2, this allows a simpler way to complete the Persona Compendium or deal with an issue when DLC Personas get in the way of the usual fusion recipe. Treasure Demons do contribute towards the completion rate of the Persona Compendium.
Treasure Demons only begin appearing from May 18th onward in Madarame's Palace and subsequently the rest of the Palaces and Mementos. When opening a shiny treasure chest, there is a small chance the shadow of a rare Persona will pop up instead and attempt to escape from the player. The chance of their appearance in Mementos can be boosted by venturing there during bad weather. There is also the infiltration item "Treasure Trap" (宝魔ホイホイ, crafting recipe from Morgana's Confidant rank 6 on July 25th) to multiply the appearance rate.
When engaged in battle, it will remain stumbled for few turns but quickly flee. Knocking it down and then asking it to join will obtain the Persona instantly. Because many of these Treasure Demons have a wide range of elemental resistances, it is recommended to rank up Shinya Oda's Confidant to unlock the skill "Down Shot" which guarantees knocking down the target regardless of resistance in exchange for firing all remaining bullets (or a portion of remaining bullets if you have ranked up his Confidant to 6). Another option is to gamble with physical skills with high critical rate (e.g. Lucky Punch) because none of the Treasure Demons except Stone of Scone block, repel or absorb physical damage.
The difference with Element demons is that Treasure Demons cannot be created via fusion but can only be negotiated and obtained in the dungeon (Element demons on the other hand never respond to negotiation).
Another important difference is that the result yielded from fusion with Treasure Demons takes the current level of another ingredient Persona into account instead of the fixed base level in fusion with Element demons. That means you can train a Persona to the desired level in order to gain different Personas of the same Arcana out of this fusion. For example, you cannot fuse Regent with Arsene of level 8 or lower because there is no Fool Persona lower than level 8 besides Arsene himself, but you can fuse Regent with level 9 Arsene which yields Obariyon who is a level 8 Fool Persona.
Fusing two Treasure Demons simply results in another regular Persona, as Mitama Personas in Persona 5 do not function the same as Mitama demons in other games. A similar mechanic of fusion with Mitama demons takes the form of sacrificial fusion (Gallow).
All Treasure Demons are modeled after luxurious jewelries or ceremonial stones instead of mythological figures.