Mix the pears, sugar, rosemary, lemon zest and juice in a large pan. Cover and cook over a medium heat for approx. 10 mins.
Add half of the preserving sugar, bring to the boil while stirring, then boil fast for 2 mins. Stir in the remaining preserving sugar and continue to boil for a further 2 mins., skimming off any foam. Ladle the jam into the clean, pre-warmed jars while still boiling hot, fill the jars to just below the rim, seal immediately. Briefly upend the screw-top jars, leave to cool on an insulating surface.