Immortelle absolute is a sweet, sticky paste made from the solvent extraction of everlasting flower Helichrysum italicuma  a small, shrubby flower prominent in the hills of Spain & Italy, named as such because of the straw yellow flowers that appear living even after dried. Helichrysum essential oil also comes from this plant but is the product of steam distillation vs. solvent extraction. Steam distillation tends to be able to extract only the most volatile components of what is being distilled and has a very different scent profile from Immortelle. Helichrysum is a warm, bracing, herbal oil with medicinal qualities.

As solvent extraction is able to pull out much heavier molecules from the plant (along with more color) immortelle is a deeply rich, sweet, spicy scent dark brown in full concentration and a brilliant red when diluted. The scent is complex and has many facets, it is sweet, root beer like. It is spicy with some eugenol/ clove aspects. It has some medicinal aspects to it, smelling slightly phenolic and lastly it is lightly floral, a soft buzzing honeyed floral similar in some ways to mimosa flower. Light, powdery and hay like. 

As a perfume ingredient, Immortelle is powerfully strong with small traces adding a deep syrupy sweetness into compositions. It is not a particularly common ingredient, I suspect as much of what it adds to a perfume can be accomplished by other cheaper materials that are more plentiful and easier to dose. The strange medicinal aspect to it is likely not a welcome addition to many straightforward commercial perfumes. It is that aspect that I personally love though, to me it feels like a scent loved by humanity but just not in this century, perhaps this millennium. It reminds of a scent would be wildly popular with the ancient Romans or Egyptians, like spikenard or mugwort which now are bizarre creatures in the vast world of scented extractions and synthesized creations.