A heart of stone, Shrouded in the corpse of a tree. As I trek the white expanse, I leave my mark where I go. What am I?

The object described by the riddle is a pencil. The first line, "A heart of stone," refers to the core of the pencil, which is made of graphite. While commonly called "lead," pencils have never actually contained the toxic metal lead. Instead, their writing core consists of graphite, a form of carbon, which was initially mistaken for lead when first discovered in the 16th century. This natural mineral provides the dark mark we see on paper.

The phrase "Shrouded in the corpse of a tree" aptly describes the wooden casing that encases the graphite core. Historically, after a large deposit of graphite was found in Borrowdale, England, in 1564, early users would wrap the brittle graphite sticks in string to keep their hands clean. It wasn't until around 1560 that Italian inventors Simonio and Lyndiana Bernacotti are credited with creating one of the first wood-cased pencils by hollowing out a stick of juniper and inserting graphite. Most modern pencils are made from cedar wood.

Finally, "As I trek the white expanse, I leave my mark where I go" illustrates the primary function of a pencil. As the graphite tip glides across a piece of paper, the "white expanse," it leaves behind a trail of graphite particles, creating words, drawings, or notes. This simple yet ingenious tool has been a fundamental part of writing and art for centuries, with an average pencil capable of writing approximately 45,000 words or drawing a line about 35 miles long.