Post HTML Issue Description with JIRA REST API v3

What kind of markup format is used?

I am using this JSON to create an issue in Jira (using v2 of the REST APIs).

{
    "fields": {
       "project":
       {
          "key": "MyProjectId"
       },
       "summary": "REST ye merry gentlemen.",
       "description": "An elevator pitch (value statement) that describes the epic in a clear and concise way.",
       "issuetype": {
          "name": "Epic"
       }
   }
}

How can I include the markup here to create an Epic with rich text as description?