Mica peridotites, the same rocks as occur in some of the diamond pipes of South Africa, can be found in kimberlite dikes in upstate New York. If you take the trail up Cascadilla Gorge on the south side of the Cornell campus in Ithaca, you can see one of these, intruded into the Devonian sedimentary rocks. No diamonds have ever been found in these dikes, but maybe you could be first! Be sure to get permission before breaking out your rock hammer.

https://www.academia.edu/19688604/Kimberlitic_Rocks_of_Centr...