Minecraft wiki entry for Development_versions, associated text: Crafted with 6 crying obsidian and 3 glowstone.
Can be used to set spawn points using glowstone as charges.
A maximum of 4 charges can be set at once, and one charge is depleted with each respawn.
Emits a slight amount of light, which gets stronger for each charge added.
Using it in the Overworld or the End causes it to explode and set fire to surrounding blocks.
When charged, a redstone comparator gives a signal depending on the number of charges.