Cool website! My boys found this in Sedona, Arizona. It is attached to the leaf. Is it an egg?
This is some type of Gall, but we are not sure what. Galls are growths produced on plant leaves, stems and other parts by usually wasps, flies and moths. They are not harmful to the plant. Inside the Gall, young insects are developing, so it is akin to an egg. The gall is usually very plant host specific. A quick internet search did not identify your particular Gall, but in time, we might have a more conclusive answer.