Ferric Ammonium Citrate (Green) 100g.   Used in the cyanotype sun-printing, blueprint process.  The Green version is more sensitive than the Brown. Also used as a food additive.
It is sensitive to daylight so must be kept in a cool, dark place.  Supplied in a resealable, tamper evident pouch which is perfect for storage.
 
The images above are examples of cyanotype, the light brown image is made by bleaching the blue image with  a solution of Sodium Carbonate and then a soaking in a Wine Tannin solution.