Solution to #2: The Letter

    The envelope includes 5 numbers which correspond to the atomic masses of Gallium, Cobalt, Francium, Niobium, and Boron. There are some more clues in the letter, but the most direct one is the peculiar stamp on the envelope. The image it uses is the logo for the ICQ instant messaging service and the contents of the letter verify that this is a step in the right direction. The letter suggests that you are looking for a number in order to message someone, and you can use an identification numbers to search for someone on ICQ so that is likely worth looking for. (Note: you do not have to download or sign up for ICQ to complete this puzzle, as you can search for users on their website.)
    The only numbers you have are the atomic masses which prove to be too long to be useful. However, you can convert them into their counterparts on the periodic table. ICQ user 312787415 has the nickname "Quartz", which is the answer.

QUARTZ