There are very few natural or synthetic fragrance notes used in commercial perfumery that start with the letter "X" ; the most notable are a few unique botanical notes and a synthetic chemical.
Xian: This is a specific note used in the fragrance Xian
by Ormonde Jayne, described by some as having a rhubarb scent profile.
Xia Xiang: This refers to a type of Chinese spice and floral scent profile found in some older fragrance products.
Xylene: A chemical sometimes mentioned in the context of fragrance compositions, though not typically a scent note that consumers would identify.
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