Round or oval shape
Bumpy, knobby, or lumpy outer surface
Hollow sound when tapped
Lighter than expected for its size
Cracked or broken areas may show crystals inside
Outer shell is harder and more solid than surrounding rock
Often found in volcanic or sedimentary rock areas
May have a dull, plain exterior with no obvious crystal features
Can be tested by gently shaking for a loose rattle inside
Cutting or cracking it open may reveal a hollow cavity lined with crystals