Pyrite suns, also known as miner's dollars or sun dollars, are unique pyrite formations found in narrow seams of slate within Illinois coal mines, particularly near Sparta. These pyrite crystals, typically disk-shaped and exhibiting a radial pattern, are believed to form under pressure within the slate seams. 
 
Location:
Primarily found in the Sparta area and other coal mining regions of Illinois. 
Formation:
They are thought to form as the pyrite crystals grow outward from a central point within the compacted slate layers, creating the sun-like appearance. 
Appearance:
Pyrite suns are often disk-shaped with a striking golden color and metallic luster, resembling a sunburst. 
Rarity:
While pyrite is common, the unique disk-shaped formation of pyrite suns is largely unique to the coal mines of Illinois. 
Mining Byproduct:
Pyrite suns are a byproduct of coal mining operations, making them increasingly difficult to acquire as coal mining in Illinois declines.