Help me identify this rock please

[sara300]

I found this rock and it has been cut in half and polished. I would like to know what kind it is. When you put both halves together it measures approximatly 11in in diameter or about the size of a softball. If anyone can give me any insight as to what they think this is or can tell me what this is I greatly appriciate it. Thanks.

[JMJones0424]

It's called a geode

 

Geodes can form in any cavity, but the term is usually reserved for more or less rounded formations in igneous and sedimentary rocks...

After rock around the cavity hardens, dissolved silicates and/or carbonates are deposited on the inside surface. Over time, this slow feed of mineral constituents from groundwater or hydrothermal solutions allows crystals to form inside the hollow chamber. Bedrock containing geodes eventually weathers and decomposes, leaving them present at the surface if they are composed of resistant material such as quartz.

[sara300]

It's called a geode

 

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I thought geodes were hollow inside? This rock is solid.

[JMJones0424]

Geodes start out hollow, but given enough time in the right circumstances, the mineral crystals can grow to fill the entire void. These are very common in areas with limestone bedrock. I fondly remember hunting geodes in the creek beds outside of Austin when I was young.