Vegepygmy
First Post
No. You just didn't look on page 220:Interesting. I didn't see where in the DMG material type changes a piece of armour into a different type. I must have an old DMG 3.5 edition.
Elven Chain: This extremely light chainmail is made of very fine mithral links. Speed while wearing elven chain is 30 feet for Medium creatures, or 20 feet for Small. The armor has an arcane spell failure chance of 20%, a maximum Dexterity bonus of +4, and an armor check penalty of -2. It is considered light armor and weighs 20 pounds.
So there you have it. Text from the DMG itself indicating that a medium armor made of mithral effectively becomes light armor.