kerno

Watch the false seam. The word looks as if it might come apart into ker- plus some -ern- ending, and it does not, because Esperanto has no -ern- suffix to strip. The root is kern-, one indivisible borrowing, and the only piece you may remove is the final -o. This is the commonest beginner error with international roots: the affix system is so productive that learners start finding affixes in words that never had any.

The meaning stretches the same way English core does. A kerno is the stone or pip inside a fruit, the kernel inside a nut, the heart of an argument (la kerno de la afero), the core of a group of people, and, in physics, the nucleus of an atom, where the compound atomkerno is standard.

Useful derivations: kerna for central or fundamental, and senkerna for stoneless fruit, which is exactly the word on the jar.