Cuendillar (pronounced KWAIN-de-yar), also known as heartstone, is an almost indestructible substance created during the Age of Legends. It absorbs any force attempting to break it, including the One Power, and is believed to become stronger as a result. Cuendillar objects have been shown to be unharmed even by balefire.
Because the making of it has been lost since the Breaking of the World, cuendillar is extremely rare and valuable. The Seanchan High Lord Turak had a vast collection of it, which both contributed to and signified his power. Cuendillar was used to make objects of great importance, including the original Domination Band and all the seals to the Dark One's prison.
Over the course of the series the Great Seals were shown to weaken, and some of them were found in pieces - something unprecedented in history. This is thought to be directly related to the Dark One's influence, because the True Power is apparently able to destroy cuendillar.
Though the actual making of Power Wrought weapons is forbidden, it is curious to note that after the rediscovery of cuendillar, the weave was not used on the weapons and armor of the forces of the Light against the Shadow. Since the weapon is already made, the Oath concerning the making of weapons is bypassed and the Aes Sedai are merely altering is physical properties, therefore not creating a weapon in its entirety. Perhaps this is due to the conception that examples of cuendillar are primarily pieces of art or tableware.
The implications of the cuendillar weave on armor and weaponry are staggering. (If cuendillar must be made from iron its weight and thickness would still be a factor.)
Armor could be paper thin and weigh next to nothing allowing suits of armor that cover the entire body apart from eye slots without over-encumbering its wearer, impenetrable to armor-piercing weaponry. There would be few practical ways to kill a soldier in this armor. Striking through the facial spaces or hitting the armor with enough force for inertia to damage its occupant are the most common ways for a non-Power user to defeat their opponent.
Siege warfare would no longer be able to utilize battering rams, catapults or trebuchets to damage walls or gates.
Weapons would never need sharpening, repairing or oiling. Hafts for polearms could be thin pipe extrusions instead of wood, thereby granting better blocking capability and mobility due to lower weight. Dragons (cannons) would be many times lighter, allowing for hand-held versions.
If the Aes Sedai had altered weapons for the Last Battle, they would probably still insist on disposing of them after it is over via dumping them off the edge of the Ways or Gateways because they could be construed as power-wrought weapons whose only purpose would be to kill men, the shadowspawn having been exterminated.
Egwene al'Vere was able to reconstruct the weave for making cuendillar from iron by using flows of Earth, Air and Fire. This allowed the Rebel Aes Sedai a source of income for their siege on the White Tower, and also became a major component of their strategy. By turning Tar Valon's harbor defense chains into single pieces of cuendillar, they prevented those chains from being raised or lowered, blocking large supply ships from entering the harbors.
After Egwene herself, Leane Sharif (Green Ajah) had the second best Talent for producing cuendillar, followed by Bodewhin Cauthon (novice) and the deceased Kairen Stang (Blue Ajah). Ashmanaille (Gray Ajah), Althyn Conly and Nicola Treehill (novices) could also make cuendillar, as well as 4-5 Yellows (from Salita's quote that half the nine sisters who could make cuendillar are Yellow).
Making cuendillar Talent
Very few channelers have the Talent to make cuendillar, for instance among the one thousand channelers herded by the rebel Aes Sedai between Altara and Murandy, only 9 sisters, 2 accepted and 24 novices showed to be able to produce this material.
Interesting facts & contradictions
The first broken seal found in the Eye of the World remained hard as in its material. When Lan tried to break one piece even more, he broke a good knife of the best steel from Tear but the piece remained unharmed.  This might indicate that it was broken by the True Power. Seals found later were not broken but one could slice off pieces of them with a knife.  The last three were broken by Logain in the Last Battle and he simply used his hand to break them.