Blue Diamonds

Unlike what their name suggests, blue diamonds are neither blue nor diamonds. They are actually sapphires. When first discovered, they were assumed to be diamonds, because, unlike usual sapphires, they were not wholly blue, but instead clear as diamonds, with a deep blue core. They kept going by that name, even when it was discovered that it was mistaken, because it had caught the people's fancy, and the people who could actually afford them, could also afford giving them a name that they fancied.
Blue diamonds are exclusive to Indice, and not even all of Indice. They are mined in the belly of the Agnello Mountains, at the north of Indice. It's not only their rarity that gives them their extravagant value, or their beauty, for that matter. Their mining is an exceedingly complicated process, as fine and time consuming as crafting a pocket watch. Each blue diamond is dug out mostly by hand. Considering that they are usually surrounded by hard rocks, this process is not at all trivial. Also the gas that is trapped on the inside of the diamonds - and which gives them their unique colour - is quite toxic, which means that, if one breaks during mining, the miner will pay with their life, and the rest of the miners will evacuate the area and not return for at least two days.
It is quite uncertain how the blue diamonds came to be. When processed in order to make sure that they won't break, the are used primarily for the creation of the tears of Jadel, pieces of jewel that symbolise eternal love. However - this is, after all, Indice - occasionally good money is paid to ensure that a tear will not be as well processed, or not at all processes. The assassins of Indice would never miss an opportunity to kill with style and elegance.