Cobalt Ore

Cobalt Ore is the lowest tier of Hardmode Ore, and can spawn on the first breaking of an Altar. More will spawn on the 4th, 7th, 10th, etc destroyed Altars, albeit in decreasing amounts. Mining Cobalt Ore requires at least a Molten Pickaxe. It is immune to all types of explosions.

Palladium Ore may generate in place of Cobalt Ore. It is slightly more valuable, and the tools and weapons it crafts have slightly better stats.

Trivia

 * Actual Cobalt is a poisonous, magnetic metal (though it is actually a key part of vitamin B12, which is necessary for living beings and has extremely low toxicity). It is named after the Kobold, at that time thought of as a mischievous spirit; this is because smelting its ores were difficult and would often release toxic fumes. Pure cobalt is an element found on the periodic table just after iron, and is actually silver-like in color, however it is never found pure in nature. Its primary historic use was as a pigment used especially in glass or porcelain, as it turns a rich "cobalt blue" when in the compound form of, made of cobalt, iron, and nickel combined with arsenic. This made cobalt, along with lapis lazuli, one of the few mineral blue pigments, and therefore was an extremely valuable commodity.
 * Several Cobalt items in the game are Japanese-themed, namely the Cobalt Naginata, Cobalt Armor and the Cobalt Sword which resembles a katana.
 * Cobalt Ore's inventory sprite is the same as the one for Adamantite Ore, except rotated and colored blue.
 * Despite being named after (and resembling) the material, the Cobalt Shield cannot be made from ore/bars.