Pemba is well known for its clove industry, and this distillery is where the clove stems are turned into essential oil. It's operated by the Zanzibar State Trading Corporation (ZSTC) and also here on occasion are cinnamon leaves, eucalyptus leaves, lemongrass and sweet basil. The tour may be a little lackadaisical, but the process is fascinating. Go first to the office and small shop selling the finished product (yes, enter through the gift shop) and arrange a guide.
The distillery is in the suburb of Machomane, about 1km north of the town centre, east of the main road. You can take a dalla-dalla to the junction then walk, or arrange a visit through a tour company.