I'm going for something simple and memorable. Let's take water, for instance. It has a simple name, and it is the most important thing on this earth to life. The sun, too. Oh, and carbon. Anyway...
This element is supposed to be allegory to oil. It's extremely precious, allowing people to increase their lifespans, others to cast magic, and it powers airships (the one JRPG element I just HAD to include). However, it only naturally occurs (we think) in the desert region of Al-Vaatir, and is hard to get. Living in the desert are nomadic tribes of elves and hostile Wyrms. Mining the stuff makes you vulnerable to both. If you've read Dune, you'll know that I'm ripping it off.
As for the stuff itself, I'm not sure what color it's supposed to be, but when it's pure, it forms geometrically perfect crystals. It'll probably be blue. Yeah, blue, the color of most mana bars on most WRPGs. When this element is alloyed with iron and/or silver, mithril is formed.
When deposits of this element are left alone long enough, the micro-organisms living on the crystals evolve into neural tissue. Over more time, this neural tissue becomes sapient. Like I said before, some of these deposits are discovered, worshiped and enshrined. One religion in particular holds that if someone is 'touched,' by this deposit (in reality, corrupted, or made into a mage), then these people have to become Clerics and go on healing missions.
Peoples bodies can be corrupted by the element if they take too much of it. Eventually, they grow dependent on it, and over more time (or if they just take a shit-ton of it over short time), their body starts to produce the stuff. Mages and clerics are in this state, but not to such a severe degree, and casting magic helps cleanse their bodies. A sign of deep corruption is a person's blood-red eyes. If they don't expel the element from their bodies, then crystal structures will start to replace their natural tissues. Their higher brain functions will atrophy. They'll be consumed with the desire to corrupt others with the element. They'll become Anathaema. My version of undead are pretty much the same thing as Cie'th from Final Fantasy XIII, if you've ever played that.
Dragons are a special kind of Anathaema that aren't really corrupted by the element, so much as they are in a kind of symbiosis. Their fire-breath is blue and leaves crystals as a residue. If anyone disturbs a deposit of the element, there's usually a dragon not far behind to protect it. This will become a plot point, later.
It has a minty scent to it. The crystal tends to be very smooth and glassy, and mithril alloys are usually shinier than other metals.