Dropped by Blue Slimes, slime crystals are incredibly valuable in the early game because of how versatile they are for crafting. Requiring explicit crafting to be created by a Guardian, they form a very staple tier for most of the game, and in a way they are good enough for beating most field bosses. They are very strong and sturdy, and they can be used for a multitude of things.
Each class has an upgraded weapon based on utilizing a slime crystal, along with that class's specific armor and the generic player-wide armors, which are the kind sold in the NPC settlements in the wilds.
Slime crystal has a slightly bluish hue to it, and it's very shiny. It's strong and resistant, most suited for blunt and slashing weapons, although they make decent weapons for those of pierce-oriented classes, but often the weapons of slime crystal, when used by pierce-based classes, serve as an alternate source for damage if needed, like if a monster is completely null to piercing abilities.
Armor-wise, Slime crystal can make relatively defense-oriented armor. The armor would be slightly heavier than the default armor, making it harder to flee from monsters, but the increase of defense more than compensates for that, offering decent resistance against shock-based damage and lessening the effects of the afterburn status ailment caused by some of the dragons in the game. Very valuable and desirable, it is also very beautiful, and with the skill of an expert Guardian, desired looks and appearances of the armor is easy to accomplish.
Other uses of Slime Crystal include the ability to use it as a sort of currency in Goblin-based NPC villages, as it is quite valuable to them in particular, and will gladly offer much more in exchange than if a player tried purchasing with the standard credits. Besides this, not much else is known of other applications due to the recent release of the open beta.