Spalted wood is from a tree that's suffering from a fungal disease, causing a web of dark lines to spread through the timber. Between the dark lines, the colour of the wood varies from segment to segment. This can turn a bland wood like beech into something quite spectacular. Here's a photo of a tree felled in a local beech wood. Unfortunately it was too big for me to slip into my pocket!
Spalted rowan.
Spalted elm.
Spalted hornbeam.
