I've asked a few scientists with backgrounds in cell culture and they have never seen this.
The structures start as small black dots, then they form a cross or X in the center, then they appear to grow like crystals. They are not in the same plane as the cells, I raise the stage away from the objectives to focus on them. I have cleaned the outside of the flasks so it is definitely inside. The structures are in all of my cultures, do not immediately seem to affect cell growth and have been present for about a week now. My cultures include different mesenchymal cell lines and use various media, the common factors are the hood, incubator and me.
Any thoughts or suggestions are greatly appreciated.