Mithril equipment is equipment stronger than black equipment but weaker than adamant equipment. All Mithril items are coloured a dark shade of blue; they are often mistaken for rune equipment because of the similarity in colour. Because of this, they are used in scams where a player will swap rune armour for mithril armour. They are also notably popular due to being lighter than equipment made of other conventional metals, while still providing decent defensive bonuses.
Mithril weapons provides medium attack bonuses. Players require 30 Attack to wield them. Mithril equipment is less expensive than black weapons, is very popular because of the significantly greater bonuses than black equipment and because black equipment cannot be smithed.
Mithril (also called "truesilver") is a fictional metal invented by J.R.R. Tolkien for his fantasy writings (Lord of the Rings, etc.). Notable is a mithril chainmail shirt worn by Bilbo and then Frodo, described by the former as "light as a feather, but as hard as dragon scales". This may not be completely accurate in RuneScape, but full mithril is lighter than (for example) steel armour, which weighs 60 kg; because of this, one could conclude that Jagex wished to make a tribute to Tolkien. In Lord of the Rings, Mithril is more like rune, than the dark blue RuneScape variant.