Zemlya stands ninth, writes z, and carries the numeral 7 from Greek zeta. Its name is the Slavonic word for earth, land or ground.
The shape is the uncial zeta, and its curved lower stroke gives it a form quite unlike the modern printed letter that descends from it. The value 7 comes with the Greek letter, which is why the ninth letter of the Cyrillic alphabet carries the seventh number: the alphabetical position and the numeral value part company as soon as any letter without a Greek model is inserted before them, and by this point in the sequence two such letters have already appeared.
For most of the tradition zemlya shared its sound with dzelo, and the distribution between them was governed by spelling convention rather than by pronunciation. Zemlya was much the commoner of the two, and after the reforms of the eighteenth and nineteenth centuries it is the only one left in the East Slavic alphabets.
The word the letter is named for does a great deal of work in the liturgy, in the psalms and in the creation narrative, and a Slavonic reader meeting the letter in a primer met a word already familiar from the services. That was the point of the naming: the letters were taught out of a vocabulary the learner already possessed from listening.