At the end of casting this spell, you magically engrave a rune on a smooth surface within range. If the surface moves more than 10 feet away from it's original location, it loses it's magic

As an action, you can crush the remaining diamond, allowing you to open a magical gateway, with one end being a point you chose within 30 feet of you, and the other being where the rune was engraved. This spell works in between planes, but has a 5% chance both gateways instead leading to random locations in the astral sea. The portal is circular, and between 10 and 40 feet in diameter. You may close and reopen the portal as an action, provided the portal has not been dispelled, but the portal remains in the same place once reopened.

The portal has a front and a back in each location where it appears. Travel through the portal is possible only by moving through its front. Anything that does so is instantly transported to the other plane, appearing in the unoccupied space nearest to the portal.

The portal lasts until it is dispelled, and is automatically dispelled after a minute.

For every additional diamond used in the casting of this spell, you may open the portal an additional time, consuming the diamond as normal. Once all the diamonds have been used, the rune remains. You may cast this spell with a casting time of 1 minute and only requiring a minimum of diamond worth at least 250 GP to create new diamond with which the portal can be opened.

The caster of this spell may use the rune as a destination for the teleport and plane shift spells as if it were a teleportation circle.