Suspendium

Suspendium (부유석, Buyuseok; or " Buoyant ore") is a miraculous mineral found and used in the Tower. The higher the purity of Suspendium, the better it floats in areas of high Shinsoo density/viscosity. The purity level of Suspendium might be one of the factors used for the ranking and grading  of certain items.

Note: the presence of Suspendium allows an item to float, it doesn't allow it to move. Moving an item is probably achieved through Shinsoo manipulation or some item mechanism.

Pockets, Inventories and Lighthouses contain different purities of Suspendium (Ch 40). Rachel's chair might also contain a certain purity of Suspendium so she can manipulate it, to move in the direction she wants it to (Ch 59).

Items with minimal purity of Suspendium are incapable of floating by themselves. A user must manipulate Shinsoo so it can float. This means the very presence of Suspendium allows an item to be able to float(by Shinsoo manipulation or by itself). Items with higher purities of Suspendium are harder to control and require a skilled Shinsoo user. So, a Regular with average Shinsoo control, is better off with an item with low purity of Suspendium.

It's possible heavier and larger items require greater amounts of Suspendium with higher purity are required to make it float. Except items like the Opera and possibly Eyes of God, all objects which're capable of floating(without Shinsoo manipulation) are smaller and lighter. Examples are: Arms Inventory, Pocket, etc.

Other examples of this is in the vehicles found throughout the Tower. A suspendicraft is a plane with suspendium in it, allowing the user to fly. This vehicle is extremely rare, due to the purity of suspendium needed. Another example of this is in the crafts used in the 28th floor by team Tangsooyook. The suspendium in here is probably low, considering the fact that the  28th floor is relatively low in the Tower. In the 21st floor, team Tangsooyook also boarded a large hovercraft. It is likely it has suspendium inside due to its buoyancy, though the effects of suspendium might be different due to the effects of water.

Urek Mazino stated that it requires at least an Opera to stop him for a tenth of a second (ref).

Trivia
The name Suspendium is a translated version and the Korean name given for this material would not be able to properly translate into English.

Suspendium