That depends on the app. Some apps send an emoji as data, others just insert something like [Emoji 45] in the text and if someone is using the same app on the other end, he'll see the picture. (An emoji is a concept, not a standard. Everyone used different ones, and different ways to send them.) It may also depend on the length oof the text. Anything over 160 characters has one of two choices - break it into multiple 160 character pieces (which some text apps handle as a single block of text, some interpret as separate blocks of text and some interpret as separate texts), or send a link to a text file on the carrier's MMS server that has the entire text. So if your text app is set to send long messages as MMS, and adding an emoji pushes the text past 160 characters, it's going to go as an MMS - data.