Aelum is an aluminum alloy the Elves developed at the end of the Slumbering Age when they decided as a culture they were too good to use iron. Aelum's composition is similar to modern Duralumin, giving it a performance slightly less than equivalent steel products, but compensated by its reduced weight and the ubiquitousness of low-level enchantments in the Elven Principalities.
Smiths of other races find the Elven attitude towards iron either silly or infuriating, but have figured out the secrets of aelum, and will use it where appropriate. Asterian noblewomen's chainmail is frequently aelum in construction.
Aelum itself is an alloy of aluminum, copper, and blood silver (chromium to modern readers). The exact mixture is a trade secret (Your GM doesn't have the metallurgy to figure it out).
Metal aelum armor has one less DEF than listed in the Fantasy hero tables, and 40% the weight. Hybrid leather/metal armor has it's metal Mass reduced by the same amount, and retains the same DEF.
Aelum's use in weapons is generally limited to light, precision blades that depend on user skill to achieve a kill, rather than weapon mass.