Confession: I am not a geologist, biologist nor physicist, so some of the terms and assumptions used here may not be correct. This is just a theorycrafting, so please do take it with a grain of salt.
It is a known fact in the scientific community of Lasklo that the gravity is not the same everywhere. There are tiny differences in gravity here and there, and it's fairly irregular. Researchers have been wondering about the reality of that for a long time, until they figured out a definite answer. It's a rare mineral they refer to as volerite. But what exactly is volerite?
Volerite (also called volitium by some, but that name never caught on in the wizardly community) is a mineral inherently magical in its nature. It is repulsed by any other material, with the sole exception of other deposits of volerite. It has a neutral attraction to itself, which allows it to cling together much as any normal material would cling to itself. Imagine like helium, but a rock - if it's not held by anything, it will just float up and away, forever to fly through the universe.This explains the irregular gravity that's present everywhere - volerite can be deep underground, but it's too weak to make any significant changes to the gravity on the surface. Well, most of the time anyway.
One note worth mentioning is that volerite's "negative mass" makes weapons, armors and objects generally made out of pure volerite... interesting. I don't feel like delving into physics, so I'm gonna move on.
Deducano
This rock forms deep underground, somewhere around the layer where magma forms, maybe slightly above it. If a volcano forms in a place where there's lot of volerite (which has a low probability to happen), there's a high chance that when a volcano erupts, it will spew volerite instead of/along with lava and ashes. Such volcano is refered to as "deducano", or "volcannon" by the commoners who think it resembles a natural cannon more than a regular volcano. When such volcano erupts, it's a sight to behold... well, if you get away from lava and ashes that is. The volerite that flies up is an interesting sight that becomes even more spectacular when some lava is attached to it. If there's too much lava, the volerite won't fly up and will instead try to force its way out of the lava before it solidifies. The rock that solidifies before that escape, trapping the volerite inside, is quite a curiosity, and not that uncommon around deducanos - it seems to be an ordinary rock, that's a lot lighter though. And it falls slower. Neat!
What's even more interesting is when a perfect mix of volerite and ordinary rock fuses together. This fusion - named “floatstone” - is apparently weightless, floating wherever one puts it. Which brings us to...
Floating islands
There's a very slim chance for a deducano to spew a nearly perfect mixture of rock and volerite, which had been fused together under the pressure even before the release. When this happens, we get what's commonly known as floating islands. Their top is covered with a layer of volerite that's trying to desperately escape the planet, and their bottom is covered with ordinary rock that's trying its best to fall. Perfectly balanced, these rocks are trying to escape forever one another, but end up just floating in the midair.
Over time, variety of things can fall onto the surface of floating islands. Water when it rains or snows, dirt and life when a tornado happens, and maybe more stuff. It's worth to mention that even though there's a layer of volerite on top of the island, the weight of the planet is too strong for all the stuff that gets caught on top of the flying island to be repulsed and thrown away. At best, it would cause the gravity on floating island to be weaker. That means that the island gets everything it should need for a life to develop. Exactly what biomes could form up there I won't detail, but I suppose the islands flying really low could have forests, while those that fly high above the clouds could have deserts because of lack of water and strong winds, arctic environment because of cold, or both at the same time.
Alloys
Since humanoids are crafty and smart creatures, they've wondered what alloys they can make using volerite. It's very hard to make a volerite alloy even for someone who is an experienced smith, especially for those that worked just on normal alloys all their life. Special tools and cooperation of several smiths is required, and only two successful alloys are known of as of now - fixite and nequitine.
Fixite
Fixite is an alloy of volerite and adamantium. Adamantium is a heavy and strong material, but also inherently magical one. When it mixes with volerite, the resulting alloy turns out to be a material that's immovable. As you may already guess, this is what infamous immovable rods are made of. But how does it actually work?
Actually, fixite is not really immovable. What instead happens is that fixite is immobile... when it gets a magical pulse of sorts. Think of it like a semiconductor - the resistance of a semiconductor decreases as much as its temperature increases. Fixite is very similar to that - it requires a minor magical pulse in order to be fixed in the field of gravity it's located in. When an immovable rod is made, this magical pulse is the button one has to press to fix it in place. It's a magical effect even smaller in power than a cantrip, and so it's useless for anything besides fixing the rod in place.
Researchers believe that there must be some way to mix the two materials to make what they call "perfect fixite", a material that will stay fixed in place without requiring a magical pulse. However, nobody has been able to produce that yet. Some artificers have begun to suspect that it may be just a false speculation.
Nequitine
Nequitine is an alloy of volerite and mithril. Mithril is light and easy to manipulate material, and - just like adamantium - mithril too is magical by nature. When it mixes with volerite, it too becomes a semiconductor-like material, but when one activates this one, it instead does not stop moving. Similarly to fixite, there is also a speculated "perfect nequitine" that would move indefinitely, never stopping and never requiring a magical pulse either.
Some of the uses for nequitine are evermoving rods, that allow ships to move indefinitely, arrows that fly at all times (until they are stopped by something sufficiently strong or break), and sources of energy.
You may now think that we've gotten to an age-old question, a paradox of sorts. "What happens if an unstoppable arrow hits an immovable rod?" Well, you see, no alloy of these metals is perfect. The answer thus depends on the smith, quality of alloy, and lots of other unseen factors. One of the two would break or otherwise give way, that's definitely given. That is also a reason why wizard society thinks that only one of the two speculated “perfect” volerite alloys can exist - because if both existed, a paradoxical state would emerge.