This vividly-named fungus is common on dead or dying oak trees–a nearly ubiquitous sight these days thanks to Sudden Oak Death. Cramp balls (Annulohypoxylon thouarsianum) look like a lump of black charcoal popping from the bark of the tree trunk. If you split one open, though, it is grey-brown and woody.
There are similar-looking fungi in the Daldinia genus, but cramp balls can be distinguished from these by the presence of tiny chicken-skin bumps.
These fungi are a telltale symptom that the tree is ill or dying; it’s often a symptom of advanced Sudden Oak Death, but can also be present on uninfected trees.