Oil
Oil -- typically olive oil -- symbolizes the good that comes from celestial love. Celestial love is love of the Lord, the highest and purest love we can have. The good of celestial love is the desire to be good springing from celestial love: Wanting to do the Lord's will because you love Him. That's not a state many reach, even in heaven, but it's a beautiful goal. This representation makes sense based on the idea that the sun -- the source of all natural light and heat, and thus all natural life -- represents the Lord, the source of all spiritual light, heat and life. Burning oil was the most pure fire available in Biblical times, thus the closest representation of the sun people could create.
Arcana Coelestia # 4192
4192. 'Gather stones; and they took stones and made a heap' means truths arising out of good. This is clear from the meaning of 'stones' as truths, dealt with just above in 4190, and from the meaning of 'a heap' as good. The reason why 'a heap' means good is that in former times before they used to construct altars people made heaps and ate on them as a witness to their being joined together in love. But later on, when the representatives of the ancients were regarded as being holy, instead of heaps they built altars, which also consisted of stones but arranged more methodically, Joshua 22:28, 34. This is why 'a heap' has a similar meaning to an altar, namely the good of love, the stones of it meaning the truths of faith.