Inspect five (1/2-pint) jars for cracks and rings for rust; discard any defective ones. Immerse the jars and rings in simmering water until jam is ready. Wash new, unused lids and rings in warm soapy water.

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.

Combine 1/4 cup sugar and fruit pectin in a small bowl. Add to nectarines in the pot; 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.

Remove from heat and stir in almond extract.

Pack 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 the rims with a moist paper towel to remove any spills. Top with the lids and tightly screw on the rings.

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.

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.