Dianoid

The Dianoid (ダイヤノイド) is a commercial building complex located in District 15 of Academy City. The primary landmark of District 15, it is described as a place for the rich and famous. It is used mostly as a shop, and is referred to as a fashionable date spot.

It is the primary setting of the 12th Shinyaku Toaru Majutsu no Index light novel volume, where it was aptly turned into a battleground by the carbon-controlling magicians known as St. Germain.

Appearance
Shaped as a hexagonal prism, the building is seventy stories high, and is lit up with a variety of colors, which change as time passes as a form of optical art. Several thick wires extend diagonally from all sides of the building, though their purpose can only be guessed as either for decoration or reinforcement. Despite District 15 being packed with many other tall buildings, these wires manage to slip into the gaps made by them.

Though it is described as being completely made out of diamond, the truth is that it is technically made out of carbon, the element that diamonds are made of. Technically, the Dianoid is made of carbon materials like carbon frames, carbon nanotubes, as well as artificial diamonds. Moreover, it's not only the buildings that is made from these materials as it also includes windows, lighting fixtures, and even furniture.

Composition
As its name seem to imply, the Dianoid is made out of diamonds, though to be more specific it is made out of carbon-based materials. According to Tsuchimikado Motoharu, the reason why it isn't made out of diamonds is that it would make the building brittle and flammable as diamonds can be set on fire. Despite this, the Dianoid is still very strong due to the carbon materials that compose it. As such, it is stated that even if a dump truck crashed into it at top speed it would still not break, and that it is tough enough to stop handgun bullets.

Indeed, so strong is the Dianoid that it would later become a detriment to both people outside and inside the building after St. Germain sealed the building with their powers, as there are no normal means, such as breaking the glass or the doors, to escape. It has also been speculated during the time Dianoid was sealed that someone could manipulate the air-conditioning system to suffocate the people inside.

For all its strength however, the Dianoid still has weaknesses due to its carbon composition. It is vulnerable to fire and high-voltage electricity. According to Stephanie Gorgeouspalace, Anti-Skill could use an acetylene torch to burn their way into Dianoid. Mugino Shizuri's Meltdowner can also burn through the materials that made up the Dianoid.

Layout
The Dianoid is the primary landmark of District 15, and is set up as an important part of the district. The Dianoid doubles as the district subway station. Moreover, the elevated highway and subway lines are laid out to cross each other like an intersection. The Dianoid has entrances in all four cardinal directions as well as a special entrance for deliveries. Because of the distance between the levels, the Dianoid uses a linear elevator, a rail system instead of pulleys, in order to prevent congestion.

Furthermore, the Dianoid also makes use of both carbon-based and organic-based DNA computers, used for their extensive security camera network.

The Dianoid is divided up into levels and each level has a specific purpose:
 * Base: At the very bottom of the Dianoid lies a graviton-style artificial gravity control device that allows the 120 thousand ton Dianoid to balance and float 10 centimeters from the ground. It is an anti-earthquake protection. This graviton-style artificial gravity control device is apparently kept secret, and with good reason as it could be used as a black hole bomb that could compress the earth and the moon to the size of a fist.
 * Lower level: There is a shopping area in the lower levels which takes up the first to twentieth floors, and where the centers of the floors have been opened up to create an atrium. The shops in this level are filled with high-class brandname shops, including White Snake and El Asine.
 * The lower level's most notable characteristic is the usage of Japanese-style or a close of facsimile of it, using Japanese-style architectural elements like sliding doors and hanging paper lanterns. Part of this facsimile of this are the floor, walls, and pillars appearing like they are made out of wood due to the carbon materials used it its construction.
 * Other areas found on the lower levels are the subway station below the shopping area proper as well as the cargo bay used for deliveries.
 * Mid level: The middle levels however are more entertainment and recreation oriented, like pools, gyms, theaters, salons, café, and restaurants that open for use. It is here that the TV Orbit station and its offices are located, part of which are open to the public, set up in a way that there is no overlap with the entertainers and production team. Away from the offices and alongside the other entertainment and recreational facilities, the station also has a viewing platform for the public to see their shows as well as the goings-on in the studio. There is also a rumor that there are hidden doors in the TV station for VIPs to pass through.
 * The mid level continues the usage of the Japanese-style construction found in the lower level. However, here, it includes a focus on water such as waterways running along the side of the corridors, water tanks embedded in the walls like windows, and decorations reminiscent of water. One of the world's largest fountains is located in this level. Likely because of its focus on water, the mid level is called the Aqua Palace (アクアパレス).
 * Upper level: The upper level contains high-class hotels and apartments, and apart from Frenda, is primarily used by celebrities as a vault or storage because of its solid construction. Unlike normal apartments, the entrances are sliding double doors, allowing for large objects like grand pianos and theater systems to pass through it. To accommodate these possibilities even the elevators of the Dianoid are large and have a huge weight limit. Naturally, a single apartment is very expensive. It is compared to a villa and costs hundreds of millions of yen.
 * The value of the apartment rides on how secure the apartment is and as such security is a central focus. So impressive is its security that it can compared to as being similar to the secret banks of Switzerland or the Cayman Islands. Because of the danger that the doors might not open if the owner loses their key, and the fact that the doors are made from nearly indestructible carbon materials, the lock makers have included a way to unlock the door in case of emergencies. Method Pin Type 29 (メソッドピンの二九式) is used for door locks.
 * These apartments also have access to the Dianoid's security camera network, apparently as a special privilege for the VIPs. This allows the customer to view the cameras that are used in the Dianoid.
 * Roof: The Dianoid's roof appears as a garden or park. The ground is covered in fine gravel, while short pine, maple, and cherry trees are planted in the area. The garden includes a small pond with a bridge as well as a bench.

Background
Prior to her death, Frenda Seivelun made use of one of the apartments in the upper level of the Dianoid as her save haven, where she can be herself and need not worry about the normal or dark side of Academy City. It is also here where she stores all the presents for her acquaintances and friends. The rest of ITEM and even Kanou Shinka were aware that Frenda did have a hideout in Dianoid however they didn't know where exactly it was in the Dianoid.

St. Germain Arc
On December 1st, Kamijou Touma takes Othinus and Index to the Dianoid after Othinus saw an advert for a doll house made out of tiny bricks, which she wanted for a place to be safe from Sphynx. Kanou Shinka, using the assumed identity of Aihana Etsu, also travels to the Dianoid in hopes of finding out what happened to his friend Frenda Seivelun. Hamazura Shiage and Stephanie Gorgeouspalace are also carrying out a job there, and the other members of ITEM are there shopping. They are then trapped in the Dianoid together with the other customers when St. Germain seals off the Dianoid from the rest of the city with his carbon-controlling Chambord.

It is not until the defeat of St. Germain that the seal on the building is undone, allowing people trapped there to finally escape.