"I do not have a stone."
Translation:У меня нет камня.
Russian nouns have several forms. «Ка́мень» is the nominative case form, it’s used for the subject of the sentence (among other things). «Ка́мня» is the genitive case form, it’s used after «нет» to express absence (among other things).
You can see all the forms in a declension table on Wiktionary: here (click on the «Declension of ка́мень» box to show it). The table might look intimidating at first, but don’t worry: you’ll learn all those forms in bite-sized chunks as you progress through the course.
This is another useful site: http://starling.rinet.ru/cgi-bin/morphque.cgi?flags=endnnnnp
Enter any form of a word and click view and it will show you all the cases.
m and ends in ь => 1st declension (PONS classification)
gs. ка́мня [ˈkamnʲə], npl. ка́мни, gpl. камне́й [kɐˈmnʲej]
*Why has the е from ка́мень vanished? Isn't it part of the stem?"
Note how the pronounciation changes in the alternative, poetic gpl. ка́мней [ˈkamnʲɪj]