Grade is a significant factor in how much a deposit is worth, and is effectively is a measure of the concentration of metal. Most copper ores contain only a small percentage of copper metal bound up within valuable ore minerals. The remainder of the ore is simply unwanted rock. Exploration companies conduct drill programs to extract samples of rock called cores. The cores are then subjected to chemical assays to determine the “grade” of a deposit. Copper deposit grades are usually expressed as a weight percentage of the total rock. For example, 1,000 kilograms of copper ore containing 300 kilograms of copper metal has a grade of 30 percent. When metal is at a much lower concentration, it may be described in terms of parts per million. However, grade is the common convention with copper, and exploration companies estimate grade through drilling and assaying. The average grade of copper ores in the 21st century is below 0.6 percent copper, with the proportion of ore minerals being less than 2 percent of the total volume of the ore rock. Investors should approach grade estimates with a critical eye. When an exploration company issues grade statements, investors should be sure to compare them to the total depth of the drill core used to determine the grade. A high grade at a low depth can have far less value than a mediocre grade consistent through a deep core.