From the The Arizona Republican of June 7, 1906:
A singular birth certificate was submitted at a meeting of the school attendance committee at Norwich Union. A widow had been called upon to prove that her daughter had reached school exemption age. She produced an egg, beautifully colored in purple, yellow and cream, whereon, in almost copperplate characters, the name and date of nativity were picked out in white, together with the texts: "The Lord shall guide thee continually," and "Teach me to do Thy will." This novel certificate was the only record possessed by the mother, and, after being much admired by the committee, was accepted as evidence.That's nothing. The November 1997 issue of the Law Society Gazette tells of a will written on an egg.
[W]ills are supposed to be in writing, but this does not mean that they must be on paper – as Lord Merrivale P found out when he came to hear a case of Barnes, deceased, Hodson and Another v Barnes (136 LT 380). There he was, sitting in his court one day, when the widow of the deceased arrived with a will written on an egg resting in a box filled with cotton wool. After the case started, his Lordship pointed out that the egg was not witnessed. He was met with the response that the testator was a mariner at sea (in fact, he was a pilot on the Manchester ship canal). The evidence and the argument went on for two days and, eventually, His Lordship gave his verdict: he condemned the eggshell will. [Link (pdf)]