Dark, Faint, Crescent shape in broad daylight.

I was out in the back yard cutting the grass and had stopped for a break. I was looking to the sky as it was a nice day, blue skies and white clouds. I noticed a dark but faint crescent shape moving fairly fast from the north west to south easterly direction. It appeared to be very high, above the clouds at least as high as a passenger jet. I removed my sunglasses and could still just about make it out. There was definitely something there. Without my sunglasses it is unlikely I would have noticed it so its unlikely many people would have seen it due to the sunlight. I called to my Wife three times but she was in the kitchen. When I glanced back it had gone. I would say I witnessed this for between 20 - 30 seconds. I kept an eye to the sky for the remainder of the day, observed many planes, birds and even a glider but nothing resembled what I had seen. I even watched some daylight sightings of the ISS on YouTube to see if that was what it was... definitely not.