This is a mysterious citrus fruit.  It exists in many different shapes, from spherical to pear-shaped, cultivated in southern Japan, it also goes by many different names names: Hyuganatsu, Konatsu, Konatsu de Tosa. Hyuganatsu a delicious fruit of Japanese origin, its name comes from the ancient Japanese province of Hyūga , (now Miyazaki Prefecture , on the island of Kyūshū, where the citrus fruit is supposed to come from, and from natsu meaning summer. 
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Hyuganatsu essential oil has the specificity of having the highest content of beta-Farnesene (woody note, rosewood) among citrus fruits. And also the very typical higher L-carvone content (cumin, mint note). This makes an amazing essential oil. The Japanese describe it as fresh sweet and exhilarating.