Place raspberries in a food processor; cover and process until blended. Strain raspberries, reserving juice. Discard seeds. Place juice in a Dutch oven; stir in sugar and lemon juice. Heat over medium-high heat to 190°. Do not boil.
Remove from heat; skim off foam. Carefully ladle hot mixture into five hot 1-pint jars, leaving 1/4-in. headspace. Wipe rims; screw on bands until fingertip tight.
Place jars into canner simmering water, ensuring that they are completely covered with water. Bring to a boil; process for 10 minutes. Remove jars and cool.
To use concentrate: Mix 1 pint concentrate with 1 pint tonic water. Serve over ice.
Editor's Note The processing time listed is for altitudes of 1,000 feet or less. Add 1 minute to the processing time for each 1,000 feet of additional altitude.