You can get the Red Meteor Gem when you kill an Alien Creeper. You can also get Red Meteor Gems from Meteorite blocks at a crash site. Note that once the meteorite block has been cooled, it will not drop any gems. However, the hotter it is, the more gems it will drop. A fortune pick is also useful to get more gems out of it.
You can craft Block of Red Meteor Gems, Meteor Timer, Meteor Crash Detectors and Meteor Summoners with the gems. Red Meteor Gems are also used for upgrading the Meteor Shield. Each gem increases the shield's range, using up to four gems.