There are two types of moonstones, the Trammel Moonstone and the Felucca Moonstone. Felucca Moonstones can be found in the Trammel lands, like the Trammel Moonstones are found in the Felucca lands. Each area is named after the two moons of Britannia, Felucca and Trammel. Felucca is the "old" lands and is considered to be the enhanced PvP area. Trammel is the "new", consent only PvP, area. Both Trammel and Felucca have the T2A lands and dungeons. You can tell on which side you are by looking at the terrain. The Felucca lands will have a devastated winter-like look, the Trammel lands will have a fresh spring-like look.

Moonstones can be found on the bodies of Bone Magi, Daemons, Dragons, Drakes, Gazers, Headlesses, Liches, Lich Lords, Lizardmen, Mummies, Ogres, Orcs, Orc Mages, Ratmen, Skeletal Knights, Terathan Avengers and Zombies. (And probably some more, yet unreported, monsters)

Initially a Moonstone has a grey color, but once clicked it says "The stone's substance shifts as you examine it." and then assumes a color that corresponds with the phase of the moon it is related to. As the phase of the moon changes, so will the color of the stone if you click on it. In moonphase order these are;
Full Moon
Waning Gibbous
Last Quarter
Waning Crescent
New Moon
Waxing Crescent
First Quarter
Waxing Gibbous
Of course a spyglass was used to see in what phase the moons were. a spyglass

To use a Moonstone it has to be inside your backpack and you cannot be inside a guarded area. When you double click the moonstone it will be placed on the ground (by you) and after 30 seconds it will sink into the ground, thus creating a colored moongate. If the location on the other side is blocked then the Moonstone will be placed back into your backpack. You may not "plant" a Moonstone while mounted.

Once the gate is open you, and anyone in your Party, can step through the moongate to get to the other lands. The moongate remains open about as long as a moongate created by a mage would. A Trammel Moonstone will lead to the Trammel lands when used, a Felucca Moonstone will lead to the Felucca lands.