This is a map of the relationship between the 8 varieties of energy that exist in our world. Water extinguishes Fire, Fire melts Ice, Ice obstructs the Wind, Wind erodes the Earth, Earth absorbs Lightning, and Lightning electrocutes Water. Light and Darkness are different from those six in that they oppose each other. These relationships have many effects on everything from magic to synthesis.
ジュリオ - Julio An alchemist with no affiliation to the guild. "People, beasts, trees, oceans and stars; they all relate to these basic 8 elements. In this world, they are all equal."
炎 - Fire The energy to ignite and consume in flames. Effects: Virus
氷 - Ice The energy to frost and freeze. Effects: Paralyze
風 - Wind The energy to cut and slice with blades of air. Effects: Silence
土 - Earth The energy to combine and transform. Effects: Petrify
雷 - Thunder The energy to electrify and break apart. Effects: Stun
水 - Water The energy to submerge and liquefy. Effects: Poison
光 - Light The energy to revive and oppose Darkness. Effects: Charm
闇 - Darkness The energy to curse, decompose and oppose Light. Effects: Blind, Curse, Sleep
Some/all of the information in this article has been culled from various FFXI guides, and translated by Elmer the Pointy.