This is a small jar of anthracite coal, buck sort, from Pennsylvania.
There is approximately 0.66 to 0.75 pound (300 to 340 grams) of coal inside the jar. There will be some slight variation from jar to jar.
Anthracite coal, or hard coal, has the highest carbon content, the greatest hardness, and the fewest impurities.
This is "buck coal". Buck is a measurement of size, not content. The standard sizing of buck coal is 5/16 to 9/16 inches (8.00 to 14.31 millimeters). Some random pieces measured 0.2 to 1.25 inches.