Shard of Archstones can be vital to the player, and can be used to get out of a sticky situation or returning to the Nexus if overburdened. They are sold by Patches, the Hyena for 5000 souls when he returns to the Nexus, after meeting him in a world, but early in the game they are hard to come by, so farming a few could be benefitial. Always keep one in the inventory, in case geting stuck in some glitched environment and unable to escape by other means.
Begin at the Valley of Defilement 5-1 Archstone, and make your way to the rats nest, directly below the Filthy Woman vendor. You can also take the shortcut that leads to the Ring of Magical Dullness to avoid a few enemies, from there you can either drop down the hole with the ladder, or carefully walk off the edge to land on the plank below. Don't jump right down into the nest below, but land on a shelf on the wall to the right first, and you can avoid the ambush in the nest all together. Make sure you have Pure Black World Tendency and the Large Sword of Searching in your off hand for better chances of a drop, your luck may effect the drop rate also. Using a bow, shoot the rats one by one, if you use spells cast a spell like Fireball or for melee use a large weapon with an AOE effect to take most of them out in one swing (Crescent Axe, Great Axe, Great Sword, Meat Cleaver). Firestorm or God's Wrath is not adviced until the Filthy Woman vendor, directly above, moves to 5-2 after you beat Leechmonger. Be sure not to attract the Giant Depraved One near her, as he may kill her, so wear the Thief's Ring. The shards are a rare drop, but you're much better off buying them from Patches, the Hyena, or learn the Evacuate miracle, which has the same effect, from a teacher.