Mystery Rock from Lake Michigan

[skye 1762]

Hi Everyone,

I live 2 miles from Lake Michigan in Zion, IL, and have started collecting rocks.

I was at the beach a few weeks ago and found a rock that has me stumped. It is almost symmetrical with similar grooves and indentations on both sides. It was the brightest and whitest rock on the beach, and was found after Tropical Storm Isaac stirred the lake up.

When backlit with a light it almost glows with the diffused light. It has a small hole in the center with crystals inside. Geodes aren't supposed to be symmetrical.

Any explanations or ideas would be welcome. I think it's worthy of treasure status!

Thanks,
Skye 1762

 

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[mamabear]

could it be a sand dollar? Harry is our resident expert, hope he can ID it. I really like it!

 

[Bajahunter]

You have an Amygdule of Chalcedony.
from this site:
Amygdules
Lava commonly contains bubbles of gas, which is no surprise because that gas is what causes lava to erupt. The empty bubbles left in the solid rock are called vesicles. Amygdules (a-MIG-dules) are what happens when vesicles later fill with secondary minerals.
Amygdules (often called amygdales by British geologists) fill with a number of different minerals, depending on the groundwater chemistry and the physical conditions underground. These amygdules are full of chalcedony or agate, but the zeolite family of secondary minerals is famous for its occurrences in amygdules. Indeed, they are usually considered to have an amygdaloidal habit.

 

[skye 1762]

Thank you for the responses! I appreciate the information and input!