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1
Inspect 5 half-pint jars for cracks and rings for rust; discard any defective ones. Immerse in simmering water until jam is ready. Wash new, unused lids and rings in warm soapy water.
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2
Measure finely chopped, peeled nectarines; you should have 4 1/2 cups. Add to a heavy pot and immediately mix with lemon juice so they don't turn brown.
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3
Combine 1/4 cup sugar and fruit pectin in a bowl. Add to nectarines in the pot and stir well. Slowly bring mixture to a full rolling boil that does not stop bubbling when stirred. Add remaining 2 3/4 cups sugar and stir to dissolve, making sure to scrape over the bottom of the pot. Boil for exactly 1 minute, stirring constantly.
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4
Remove from heat and stir in almond extract.
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5
Pack nectarine jam into the prepared jars, filling to within 1/4 inch of the top. Run a clean knife or thin spatula around the insides of the jars to remove any air bubbles. Wipe rims with a moist paper towel to remove any spills. Top with lids and tightly screw on rings.
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6
Place a rack in the bottom of a large stockpot and fill halfway with water. Bring to a boil and lower jars 2 inches apart into the boiling water using a holder. Pour in more boiling water to cover jars by at least 1 inch. Bring to a rolling boil, cover, and process for 10 minutes.
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7
Remove the jars from the stockpot and place onto a cloth-covered or wood surface, several inches apart. Let rest for 24 hours without moving the jars. Gently press the center of each lid with a finger to ensure the lid does not move up or down. Remove the rings for storage and store in a cool, dark area.
