Your piece isn't entirely unlike rhyolite. I was thinking Marblehead rhyolite, but your piece also looks glossy, and few rhyolites will be glossy. We also have various Hudson Valley cherts that show up in our region. All that said, it does look like a D shaped gun spall. The natives used both European made gun spalls/flints and they also manufactured their own. Your piece would be European made, however. It does not have to be native associated; it could be settler related. But I do not know the site you are finding your recoveries on, you know that best and are in a better position to make the call as you get to know the site.
Here is the only other such find I have, a square gun spall, also called a rectangular gun spall. An expert in the subject suggested 1750-1850 for this. I did find it on a site that is 95% Late Woodland, with mostly quartz Levanna and quartz Madison points. Also found an 18th Century kaolin pipe fragment. 17th Century blockhouse and trading post nearby, and I do believe the site was occupied when the first settlers moved into the town just a stone throws distant.
At any rate, here is an example of a European made square gun spall. And BTW, the book you ordered by my late friend and colleague, Jeff Boudreau is one of the very best regional guides in print now, and will be the Bible on the subject for our region for years to come. You will also want to pick up the MAS guide to artifacts from southern New England to serve as a great guide to artifacts other then projectile points....