Rhombus, or rhomb (plural: Rhombuses or Rhombi), is a very popular socket stone and can only be used in weapons. Rhombuses/Rhombi increases the strength of the weapon, which means the wielder is much stronger. There are several kinds of Rhombi, ranging from +5 damage and all the way up to +44, +52 etc. Rhombi are mostly used by melee (It needs to be non-magic/ranged as it does not increase magic damage (that's the Pyramid's job)) users, like Azure Knights and Bagi warriors.
Rhombi are available even for level 1 players, but can be harder to obtain. Usually, they get dropped by a special monster named Akris, which is like a giant glowing deer. +33 Rhombi are available from Akris in North Ares, North Morte, Deneb(also called Denebe), Requies, Heiharp and Norak & Castor 1st floors. Areas other than these usually have +44 and up, depending on the current Map (level) the player is on. Crespo is the highest level area where there are Akris.
Some monsters also drop Rhombi, but they are much more rare and have a lower potential. For example, in an area where there are Akris dropping +44 Rhombi, the monsters might drop +11 and +15 Rhombi only, with a 1% chance only. This makes Akris Hunting very popular.