This piece is about 2.5 inches long and 1.75 inches wide and is as thin as 2 quarters. 
Akro Agate cullet is crushed glass from Akro Agate Company, a historic American glassmaker best known for its marbles and agate-style glass.
The glass has a distinctive swirled, stone-like pattern and is commonly used for kiln work, casting, mosaics, and jewelry.
This cullet contains uranium, which causes it to glow bright green under UV/blacklight.