String is the 1st item unlocked in the Silk Store. The Silk Store is a Regional Commercial building that is unlocked at Level 25 and requires the Limestone Cliffs region to be unlocked.
String can be produced in 22 minutes in 2 stages without using the Global Trade HQ or a Speed-Up Token. Based on this production time, the simoleons/minute is 5.45. (High-tech Factories and 3-star shops were used for these calculations.)