For follow-up discussion and general commentary on the topic. Comments are sorted chronologically.
I think one should take into consideration where Jesus was at the time and when he actually started writing on the ground. First it says he was in the temple so the floor in the temple would have been stone. Next he started writing only after they asked him the question in relation to the law.
I can only speculate that he was trying to show that He gave the law and they were trying to trick the giver of the law. However this is only speculation.
In the end the focus is not on what was being written on the ground but a host of other lessons.
Jesus was teaching when the leaders came to him with the lady. They accused her of adultery and said that the law required that she be stoned to death. Jesus then knelt and began writing in the dirt. What he wrote is not written in any of the gospels. What was written was not important. It may have been that he was writing a list of the sins of the men standing there. The Bible says that the men, from the oldest to the youngest, left. When the last one left, Jesus asked where her accusers were. She said they were gone and Jesus told he that he did not accuse her either and told her to go and sin no more.
The lesson here for all of us is that we see others' sins much easier that we see our own. When we take our time we can see our sins also and know that we are no better that anyone else. Our calling is to love each other and encourage each other just as Jesus did when he did not condemn the lady but loved her and encouraged her by telling her to go and sin no more.
#Adultery - just letting those around Him know He knew who they really were. OK, so they didn't have Twitter yet, but I'm sure that it was something they all could see and understand that He knew their guilt.