Non-sensitizing plant extract from the gourd family (think pumpkin) that’s often reputed to be useful for improving the look of puffy eyes. Claims of cucumber having skin-soothing properties are anecdotal, as there is no research to support this contention.
Most types of cucumber are composed of 95% water; the other constituents are primarily ascorbic acid (vitamin C), caffeic acid (an antioxidant), the mineral silica, plus other trace minerals. Like most plants, cucumber contains chemical constituents that have some amount of antioxidant activity. Cucumber does not contain fragrant components known to be sensitizing to skin. 
- Ibrahim T, El-Hefnawy H, El-Hela A. Antioxidant potential and phenolic acid content of certain cucurbitaceous plants cultivated in Egypt. Nat Prod Res. 2010;24(16):1537-45.