The (fixed or non-volatile) oil coming from the black seeds of Nigella Sativa, a smallish (20-30 cm) flowering plant native to Southwest Asia. The seed has a very complex chemical composition (it contains both fixed and volatile oil) and is used traditionally for a bunch of "anti-something" abilities including antitumor, antidiabetic, antihistaminic, anti-inflammatory, and antimicrobial properties. In Islam, black cumin seed was considered "a healing seed for all diseases except death”.
As for modern research and chemical composition, the fixed oil from the seeds is rich in skin-nourishing unsaturated fatty acids (mainly linoleic acid at 50 – 60% and oleic acid at 20%, but also contains some rare ones like C20:2 arachidic and eicosadienoic acids), amino acids, vitamins, and minerals. A component called thymoquinone (it's the main component of the volatile oil part, but the fixed oil also contains some) is considered to give the seed its main therapeutic properties including strong antioxidant and anti-inflammatory abilities.
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As for black seed oil and cosmetics, the oil is great to nourish and moisturize the skin and is highly recommended to treat inflammatory skin diseases like psoriasis and eczema.