The easiest way to acquire Nanotubes is to decompose Diamonds in the Chemical Decomposer, but a better choice is to use the Chemical Synthesizer to craft 256 Carbon (C) into 1 vial of Carbon Nanotubes.
Carbon Nanotubes can be used in the Chemical Synthesizer to create Diamond Dust (which can be processed back into a Diamond), Flux Crystal (with the addition of Iron (iii) Oxide and Copper), or a Block of Diamond. 3 Nanotubes will make one Diamond Dust; 27 will make a Block of Diamond.
Carbon is not hard to acquire—every organic substance in Minecraft contains Carbon in ample quantities—but it still requires 6,912 Carbon per Diamond Block. Charcoal is a good source of pure Carbon, however with an expected value of 6.56C/charcoal (82% chance for 8 C) it requires about 1,054 charcoal per diamond block, which will also consume at least 2.1 million RF to decompose (1054 charcoal items * 2kRF/item).
Cellulose (C6H10O6) is another source of Carbon. About 288 Logs will yield 1,152 Cellulose, which will decompose into enough Carbon for a Block of Diamond. Wood logs yield an expected value of 4 cellulose per Log (50% chance for 8 cellulose), which means it will take at least 2.8 million RF to decompose (288 logs * 2kRF/item). This will also yield 11,520 Hydrogen (H) (H) and 6,912 Oxygen (O) (O), which will need to be removed from the Decomposers and stored or destroyed.
Cucurbitacin (C30H42O7) comes from Pumpkins. 231 Pumpkins yield 231 Cucurbitacin, which is enough Carbon for a Block of Diamond. It will take about 0.9 million RF to decompose (231 Pumpkins * 2kRF/item). This will also yield 9,702 H and 1,617 O, which will need to be removed from the Decomposers and stored or destroyed.