Lemon Plum
A lemon plum is a rare fruit imported from Chile. It resembles the taste of a plum, and is very sweet and juicy. The fruit has a similar shape to that of a lemon, except one end is rounded and the other end has a point. When unripe, they have a bright yellow color, and are sour. As they ripen, their color changes from yellow to a golden orange and then finally to a dark sunset reddish purple when ripe. They have a slightly floral, very fragrant smell. Their peak availability in the United States is very short and is typically within the month of February.
It is very likely that this is the plum variety called Inca, introduced by Luther Burbank in 1919 and grown in very limited quantities in the United States. L. E. Cooke Co., a wholesale grower of bareroot fruit trees for orchards and home gardens, lists it in their catalog: http://www.lecooke.com/cms/le-cooke-blog/Inca-Plum-History.html It has a low chilling hour requirement, making it suitable for coastal areas and Southern California, and ripens in August in Central California, hanging on the tree until the end of the month.