I received a Rune Shard. What is it used for?

Rune Shards are parts of a Rune Circle that is required to build Ancient Wonders. Before you can start to build an Ancient Wonder, you need to collect the 9 unique runes of the specific Ancient Wonder to complete the full rune circle. In some cases, the magic process will just try to reveal a Rune which has already been activated and in this case your Shard will break and become a Broken Shard. With ten Broken Shards, you can get an guaranteed unlock of any Rune in any of your Ancient Wonders.