Resonant Gems

Resonant gems are the magical crystals which hold up the islands, which become weightless when exposed to sounds of a particular octave; they are also responsible for the "stars" visible in the void on a moonlit night. On the most basic level, they convert other forms of energy (light, kinetic, friction, electricity, etc) into heat and magical energy. 
In nature, resonant gems are most abundantly found on the bottom of floating land masses, but can also be found underground. There, they resonate with a sound which emanates from the void, which is fairly soft but far higher than the range of human hearing. Miners who excavate crystals from the bottom of the islands report constant headaches, and it is believed that exposure to the sound for long periods of time has caused delusions and, in some cases, madness.
Furthermore, they magnify the magical effects of moonlight, causing it to reflect off resonant gems several times brighter than the source itself. In nature, this phenomenon causes loose resonant gems floating above the void to appear as stars, as if the sky were both above and below you.
In artificial applications, the crystals have a wide variety of uses.

Artificial Uses

When attuned to a certain instrument, the gem will become particularly sensitive to resonance with the sound of that instrument; this is frequently used by Bardsto add attack damage to their ally's melee attacks, or to make their weapons lighter. It is also used in the transportation of cargo.
When cut into a suitable shape, resonant gems can also be used to magnify moonlight in a specific direction, creating a concentrated beam of magical energy in a specific direction. Because moonlight has such raw magical potential, a gem cut to condense this energy will shatter if exposed to moonlight for more than a second; after this, they require approximately 30 seconds to cool off before being exposed again. When combined with a moonstone reflector, the light from a small flint striker can be amplified and concentrated to create light rifles. 
Additionally, they can be ground up to create resonant powder.

Rarity

Technically very common, but the difficulty of mining / harvesting it makes it very expensive.

Base Price

~1 Real / ounce