Rubedo

"A jagged crytalline [sic] ore. Gives off radiant energy."

- In-Game Description

Rubedo is an uncommon component that can be found on Earth, Pluto, Europa and Sedna regions. It is usually found in quantities of 10 to 50. Although Rubedo is an uncommon resource, most recipes that use it require extremely large amounts, making it the most farmed resource.

Gathering Tips
These are based on opinions and may not be 100% true. These should be viewed as advice for finding the materials until better facts are proven.
 * Earth Major looting locations include Oro, Pacific, and Gaia. It drops in the normal quantities. The infested maps seem to be best for farming as the vast amount of enemies thrown at you can be killed and farmed for Rubedo along with the chests and lockers around the map.
 * Note: The most productive way to farm rubedo would be found on Pacific. Go solo or with a team mate and slowly check every locker, break every storage container, and make a lot of noise to bring the infested to you. This should gather around 90+ rubedo each game. (Tested 3 times with successful results)
 * Unsure if above author is posting before or after update 8. As of June 7th was tested 4 runs receiving between 16 and 60 rubedo per run. (If unlucky, good runs can net you over 100 rubedo if the enemies decide to drop some as of 6/21/13).
 * The Europa map, Valefor, drops a high amount of Rubedo but requires a higher level than Earth maps. You usually recieve 150-200 rubedo per run. Tested 10 times with successful results, and only one run at 126 rubedo. (Suggestion: Ask others to mark rubedo)
 * On Europa, the Defense mission, Eligor, also has large amounts of Rubedo. A high level is required though.
 * The most productive way of farming Rubedo and Alloy Plate--regarded as two of the most farm-intensive resources--is in Sedna with a full party in the mobile defense mission Kappa. Hordes of enemies appear during a 4 player run--bringing with them many materials/mods and XP earnings. It is recommended to include a Nyx in your team and/or frames with AOE abilities to speed up the process. Another good team build is with a Frost and Banshee. Have Frost drop a Snow Globe around the Data Terminal to prevent enemy fire and to slow down any Grineer and Rollers that march up to the Terminal. Banshee with maxed Focus and Reach mods can spam Sound Quake, effectively clearing out whole rooms, as energy is not really a problem with the large amounts of drops that occur.
 * Now with the new update of the pre 7.10 8 saw a huge increase in quantities of Rubedo in Europa with many missions having 2 or more drops in quantities of 10 to 50 units.