A very interesting rock - Any guesses

Marlborough, MA(Zone 5b)

I discovered a rock of the kind I have never seen this summer while planting some flowers.
Now I don't live in any special geological area such as near the coast or a mountain or out west near a desert area.
It is a suburban area half way between Boston and Worcester in Massachusetts.
Anyone have a geological background and can tell me something about it?

Thumbnail by dogwalker
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