A dense essence block is a raw material used in the Runecraft skill to make blood runes, soul runes, and Arceuus spellbook teleport tablets. It is obtained by mining at the dense essence mine, granting 12 Mining experience and 8 Crafting experience per block. Dense essence blocks can be used on the Dark Altar (requires 100% favour) found north-west of the dense runestone mines, to infuse them with dark power, turning every dense essence block in the player's inventory into dark essence blocks. One Prayer point is drained for each block converted, however players can still convert blocks with 0 Prayer points remaining.
These dark essence blocks can be crushed with a chisel to turn into dark essence fragments. These dark essence fragments can then be used for runecrafting blood and soul runes. Alternatively, the dark essence blocks can be brought to the lectern located in the Ouditor's house in Arceuus to create teleport tablets.
Dense essence blocks and dark essence fragments cannot be banked, however dark essence blocks can.
Another use for these blocks (though not recommended) is giving them to Clerris to gain Arceuus favour. This method of gaining favour only grants 0.1% per block donated, versus 2.5% per book or 5% per manuscript donated in the library. However, this method does remove the constant running required for the other methods and could be of use to people with lower Agility levels.
Players cannot get the rock golem pet while mining dense essence blocks.
The only way of obtaining dense essence blocks is to mine them at the dense runestone mine, located in northern Arceuus. Both a pickaxe and chisel are required.
Mining and Crafting levels of at least 38 are required to mine dense essence blocks, granting 8 Crafting and 12 Mining experience per block mined.
After completion of the medium tasks in the Kourend & Kebos Diary, players have a 5% chance to mine two dense essence blocks at once. No additional experience is awarded for the extra block.
Players will always get dense essence blocks every nine ticks, regardless of the pickaxe used. Having a high Mining level is slightly advantageous when mining the essence, as the chance of the runestone not depleting scales between 85% and 90% from 38 to 99 mining.
For an exhaustive list of all known sources for this item, see here