Would you happen to be near Patapsco State Park ? That specimen looks like feldspar from theFany Frost quarry . If so ,the next time your out there check the road going into the quarry for large diopside. They will be rectangular and dark blue--green ,and opaque.  There is also a report of large titanite .Further down stream on the Patapsco there is the Davis tunnel, directly over the tunnel's entrance there is a large amount of yellow/blue sphene.