Eye of Ender

An Eye of Ender (or Ender Eye) is a craftable item used to locate and activate end portals within strongholds.

Trading
Eyes of ender can be obtained by trading with cleric villagers, and cost between seven and eleven emeralds each.

In 1.9, clerics will no longer offer to sell eyes of ender, and will sell ender pearls instead.

Locating strongholds
Eyes of ender can be used to locate and activate end portals, or to craft an ender chest.

To locate end portals (and the strongholds that house them):
 * Pressing while holding an eye of ender will cause it to fly in the direction of the nearest stronghold, traveling through any blocks necessary, and leave a trail of purple particles in its wake.
 * In the PC edition, the eye will lead to the portal room when the stronghold is first generated in the world. If the world is exited and reloaded, the eye will instead lead to the spiral staircase that was the first room generated.
 * In the Console Editions, a thrown eye will lead to a stronghold, but not exactly to an end portal.
 * While far away from an end portal, the ender eye always travels upwards to offer an easily-visible indication of the horizontal direction the player must travel.
 * When the player gets close to the end portal's horizontal coordinates, the eye floats in a more precise direction that takes the y-axis into account. This often means the eye will travel downwards, to indicate the player is above a Portal and must mine downwards.
 * After two or three seconds of travel, the eye floats in the air briefly, then either falls (becoming collectable again), or shatters in mid-air. The Eye has a 1:5 chance of shattering.
 * The Eye of Ender's flying function only works in the Overworld. It does nothing in the Nether, the End, or in worlds with no strongholds.

Activating end portals
Once an end portal is found, eyes of ender will also be required in order to activate it. End portals require a total of 12 eyes of ender in order to activate, though each individual frame block has a 10% chance of containing an eye of ender when generated. Eyes can be placed in empty End portal frames by pressing on them until the entire ring of 12 is filled, thereby activating the Portal.

Trivia

 * When in 3rd person view, the eyes of ender fly out from the player's feet instead of their hand.