Type "rune uncommon" to get a list of uncommon runes that you are able to draw on a runestone. Type "rune teach" to see which of them, if any, you can teach. To gain the ability to teach an uncommon rune, you must win a raffle of otherwise acquire it at a merchant event. Even then, you will usually have the ability to teach it only one to three times, and sometimes zero times.

Knowledge of uncommon runes (by which I mean the ability to draw an uncommon rune on a runestone) is not nearly as valuable today as it was when the the minor summoning spell was initially released. Prior to HSN 2015, the only way to summon a particular type of demon was to use a drawn rune or to get lucky through random summoning attempts. During HSN 2015, the minor summoning spell was changed to give everybody a chance, based on skill, to summon a particular type of demon without the use of a runestone. Drawn runes are still useful because they greatly increase the chance of success.