The shamir is a subject of Jewish lore that is described as either a substance or a creature that can cut through nearly anything
Appearance
While originally described as a green stone, the shamir has also been interpreted as an herb or a living creature, most often a worm. It is the size of a grain of barley. The only description of its colouration in worm form describes it as white, but it is implied to be a distinction from an ordinary shamir.
Behaviour
The most notable trait of the shamir is its ability to bore through any substance besides lead. Strangely enough, it never actually digs or cuts into anything; instead, it is variously described as touching, gazing at, or bleeding on things in order to break them. Some accounts even claim that the design needed to be drawn before the shamir could do anything. It seems to have some control over the degree of this ability as it is capable of both splitting substances and engraving them. It has also never been described damaging anything living, which implies that either the holder or the worm has control over what it cuts.
Because the shamir can disintegrate most containers, it must be wrapped in wool and then stored in a lead vessel. Occasionally the vessel needed to be filled with barley as well.
According to John Martin Woolsey, the shamir also "bursts locks, opens mountains, and restores life", but this appears to be among the only mentions of such abilities.
Cultural significance
Mentions of the shamir are scant, but generally it is used in place of iron tools because they can also be used for war, unfit for building anything to promote peace. Most notably, it was said to be used for Solomon's Temple in Jerusalem and to engrave the breastplate of the high priest. The substance was the seventh of ten marvels created by the Abrahamic God on the first Friday, which was what lended it its supernatural properties.
The shamir was also reported to carve gemstones with magical symbols. This led to a belief that engraved gems had mystical properties and an association with a guardian angel.
One theory as to the nature of the shamir is that it is a radioactive mineral. Prolonged exposure to the alpha particles would discolour or weaken many substances, and lead would indeed be a safe way to keep such minerals.
Related myths
The most prominent myth involving the shamir is the story of Solomon, a king of Israel, and his acquisition of it. He needed the shamir in order to build the first temple in Jerusalem, which could not be hewn with tools of war. He was first told to visit the demon Asmodeus, who then informed him that the shamir was not in his possession, but rather in the hands of the Prince of the Sea. The Prince of the Sea, however, had given it to a bird called "rock-splitter" (often translated to mean wood grouse, rooster, ostrich, or hoopoe).
In order to get the shamir, the bird's nest was covered by glass. When it returned home the shamir shattered the glass, and one of Solomon's assistants leaped out to startle the bird. After they had taken the shamir, the bird, ashamed that it had failed to keep it safe, strangled itself.
By the time Jerusalem's second temple was built, the shamir had vanished, having served its purpose.