Storm Gem

Shining crystals filled with pulsating sparks. Found where lightning strikes the sea.
Most storm gems slowly sink towards the seabed, forming a critical part of the ecosystem in more storm-aligned waters. Numerous coral-like organisms rely on them for energy. Once drained, the gem rapidly expands and blackens, becoming Stormglass . This is how deposits of stormglass build up- gems are naturally drawn towards existing bodies of glass, resulting in areas clumping up. The biggest concentrations of stormglass exist around Whirlpool, an area famous for its constant thunderstorms.
Jewelry made from storm gems generally involves encasing the gem in glass or crystal to prevent energy leakage.
Note that you can't simply cast lightning bolts at the sea to create storm gems. Actual lightning bolts (and not just standard lightning magic projectiles) cast will generally result in very sub-par and volatile storm gems; this can be used to generate stormglass. Now, if one used magic to generate the storm itself, they'd be able to make standard-quality storm gems. It's no easy task, and more than one Whirlpool binder has met their end attempting it.

Deposit mechanisms

'smooth' deposits of stormglass are created because of two main factors. First of all, storm gems are naturally drawn towards existing concentrations of gems and glass. While this mostly boils down to correlation (the most likely spot for a gem to land is where gems will cluster, typically funneled there by whirlpools), there does appear to be some sort of effect which further exacerbates this. Second, the energy generated by storm gems naturally causes stormglass in contact to 'melt', smoothing out the deposits and fusing them into a single piece.

Storm gems & magma

While the massive stormglass deposits around Whirlpool are more or less expected, finding stormglass beneath the magma flows of the Shattered Jungle is an obviously unnatural occurence. The origin of these deposits actually stems from an ancient monster's massive hoard- at some point, some individuals redirected a magma pipe into its lair in attempt to kill it. This is also the source of a few other esoteric materials which ordinarily would have no right being where they were.

Background physics

Storm gems are composed of silicon dixode, though the material has no no perfect real-world analogue because its molecular bonds held together partially by magic. Once enough magic dissipates, it decays into a stable state very similar to obsidian. Trying to 'rehydrate' the material with magic causes these bonds to reform (and in some cases, replace existing bonds temporarily) without the necessary structure around them, reducing the rigidity of the material.
By definition, storm gems exist in an extremely unstable state which gives off excess energy until settling into a more stable structure (stormglass). In their charged state, they are far denser than otherwise; part of the process of losing energy also causes them to be less compressed. There's likely some degree of Loose superimposition involved in the process, rather than it being pure density in the conventional sense.

Uses

Stormglass crafts

Storm gems can be drained in a controlled environment (i.e. a mold) to make the resulting stormglass conform perfectly to a given shape.

Transmutation

Storm gems are essentially bundles of Storm-aligned latent energy. They can be converted into various primitive storm spirits rather easily.

Other applications

Storm gems can be shattered to release electricity- grinding the material (e.g. with a belt sander) allows one to do this in a more 'controlled' manner. This is used to fuel electrolysis in the refinement of aluminum ores.