Ferrite

"Alloy pellets used in Grineer manufacturing."

- In-Game Description

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

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.
 * Ferrite can be effectively gathered in large quantities in Neptune, especially on the Mobile Defense Mission Larissa. Neptune has a rare element which in turn means it is more likely the other two elements are going to be dropped when a resource is rewarded. This contrasts with Earth, which has a Mobile Defense Mission but no rare element.
 * In addition to Larissa, the endless defense mission . While Larissa is a mobile defense mission, the amount of mobs spawned in it are much lower than that of Sedna's Kappa. The endless defense mission 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, but this time with infested. There are also tons of containers, and a speed run to wave 5 can yield 200+ ferrite in a matter of minutes.
 * Lua (Mobile Defense) of Earth is good place for farming too, as a combination of Exps and Ferrite.
 * M-Prime (Mercury) is very good place to farm ferrite in 8.0 update. You can gain 400-600 ferrite per run without full level exploration, just running mission objective, opening chests, breaking footlockers and killing enemies (they drop ferrite too). Would be best to equip Thief's Wit so you don't waste time looking for chests.
 * Despina, also on Neptune, is also a very efficient place to be farming for ferrite. Theoretially you could get 2k+ ferrite in one run. The chance of this is very very small as that would require each breakable chest to drop 100 ferrite but it is 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).