Balanced Shards are created by combining any of the six Primal Shards with two each of every other primal aspect. This can be done in a Crucible or automated alchemy device. They are used in the creation of Salis Mundus and Primal Charms.
For example, throwing in 2 Cobblestone (1 Terra, 1 Perditio), 2 sugar cane (1 Aqua, 1 Aer, 1 Herba) and 2 Blaze Powder (1 ignis). This gives you 2 of each primal aspect except Ordo, so throwing in an Order shard will complete the process.
If you have access to the Outer Lands, mining the (rather common) white crystal clusters on the ceilings and walls will give 1 Balanced Shard each.