Ferrite

"Alloy pellets used in Grineer manufacturing."

- In-Game Description

Ferrite is a common component that can be found on Mercury, Earth, Neptune, and Orokin Void. It is usually found in quantities of 50 to 100.

Blueprints Requiring Ferrite
Click to view the Blueprints requiring Ferrite. 

Market
"By purchasing this item you can directly exchange 30 for 3000 Ferrite."

- In-Game Description

Beginning in Update 9 materials could be purchased directly from the market for.

Trivia

 * The Corpus letters etched on the picture transcribes to Fe, the chemical symbol of Iron.
 * Ferrite is a scientific name for alpha-iron or pure iron. It is used in carbon-iron alloys.
 * Ferrite's icon appears to be the icon for Gallium, but with a different texture.

Gathering Tips
These are based on opinions and may not be true. [These should be viewed as advice for finding the materials until better facts are proven.]
 * After Update 10 the easiest and fastest way to farm Ferrite is Apollodorus on Mercury, about 1,000 ferrite for every 5 minutes.
 * Eurasia (in Earth, Interception) 700 - 1,000 drop. It is an extremely easy way to obtain Ferrite, as enemies are low leveled.
 * Caloris, Mercury was best place to gather Ferrite prior to Update 10 -- normally getting between 200 to 1,000 a run. Since this level is a low-level exterminate mission with around 50 enemies, you can easily clear the map out, loot the lockers and containers in short order. This allows you to rack up a huge amount of Ferrite quickly.
 * Ferrite can be effectively gathered in large quantities in Neptune, especially on the Mobile Defense Mission Larissa.
 * Thalassa has a large number of storage lockers (although some can't be opened without master thief) . While Larissa is a mobile defense mission, the amount of mobs spawned in it are much lower than that of Sedna's Kappa. Thalassa has a higher concentration of mobs and also contains a high amount of storage containers and lockers, which also have a chance to drop ferrite.
 * Also, Proteus in Neptune has a high drop rate of ferrite. Another endless defense. There are also tons of containers, and a speed run to wave 5 can yield 200+ ferrite in a matter of minutes.
 * Lua (Interception) of Earth is good place for farming too, as a combination of Exp and Ferrite.
 * Despina, also on Neptune, is also a very efficient place to farm Ferrite. You are likely to get near 500 ferrite on almost every run.(Confirmed)
 * Tolstoj on Mercury. After you confirm your presence to the enemy, find safe spot, ALT+TAB, go back after 5-10 minutes, kill all, repeat - profit! Alternatively, just clean whole map without waiting. Every third resource drop is ferrite (70-92).
 * Post Update 9, M Prime is still a viable place to farm Ferrite. It will yield anywhere from 400 Ferrite minimum to over 900 a run. Break open all the containers and open all the lockers to achieve maximum possible yield. Master Thief will be of help in achieving as much loot as possible. (Confirmed)
 * Terminus on Mercury is also a good place to gather Ferrite. Open every container and locker, and you should end up with anything between 300 and 700 ferrite. Low level enemies in little numbers make the mission easier and shorter.