Cunedillar is a material made using the One Power that is considered unbreakable. Any force directed against it is said to make it only stronger and even balefire will not destroy it (TSR, Ch. 54). The making of it was lost in the Breaking and all examples date from the Age of Legends (TEotW, Ch. 52). There was some knowledge of it during the Breaking however, as the Domination Band was made of "a form of cuendillar" (TSR, Ch. 54)
It is rare, and discoveries of troves of it have been known to make the finder rich (TDR, Ch. 48). The weave to make it has recently been rediscovered, by Egwene al'Vere, whose natural ability with Earth was no doubt an advantage. It was accomplished by a complex weave of earth into iron, with few Aes Sedai having sufficient strength to perform it well (CoT, Ch. 17; Ch. 30). There was at the time some debate amongst Aes Sedai over making it to raise finances (CoT, Ch. 17). Though considered unbreakable, the seals on the Bore into the Dark One's prison, made of cuendillar, have decayed and broken (TEotW, Ch. 52). It is also known that the True Power can destroy cuendilar (TGS, Ch. 22). The Aes Sedai believe they have a full record of all cuendillar ever made (TGS, Ch. 5).
No known force could break cuendillar once it was complete; even the One Power directed against heartstone only made it stronger (The Gathering Storm, Chapter 5).
If everything on those shelves was truly cuendillar, it was enough to buy a kingdom, or at the least to found a great House. Even a king might beggar himself to buy so much of it, if he even knew where to find so much (Domon, on Turak's collection (The Great Hunt, Chapter 29)).