Triangle black craft

I was at work on a break when I saw something out of the corner of my eye. I looked in that direction but only got a split second to see it because it flew behind a tree line. I knew then it was something unusual. I assumed it was a farmer's plane and kept looking to see it come out from the other end of the tree line. Instead, the object came out at a 90-degree angle. When I got my closest look at it, it was about 50 yards away from me. This is what I saw: it was flat, maybe 20 feet in width, possibly smaller, and about 10 feet in height, maybe shorter. It started to rotate in an unnatural way, and when it did, I saw that it was in the shape of a triangle, but in the middle of the triangle, there was another triangle inside that was see-through. The color of it, the best I could describe, is black as black marble and shinier than marble. It looked like all the material was one piece, like no rivets, no seams, just like the craft was made by one piece of whatever material it was. The sun reflected off of it. It made no noise, there were no propellers, no propulsion; it was just floating and rotating perfectly with no disturbance, almost gracefully perfect in its rotation. Then it continued to do that as it slowly flew away until I could not see it anymore, rotating gracefully the whole time. There were two other employees with me; they only witnessed the end part of it but could still see it rotating because I called them out to come look at this. I wanted to take a picture of it but I didn't want to take my eyes off of it and miss something.
It started off to the right of me then took of slowly into the air directly in front of me then flew away