By Unclebob
United Kingdom 
Have you any idea what this fruit could be? It's growing on the Costa del Sol but is falling from the tree before it ripens. Its size is between a ping-pong ball and a tennis ball. The skin is hard and the white inside is like polystyrene (although it is unripe). We've discounted gages, figs, limes, chirimoya, quince and a few others as it only has one central stone. Could it be a nut? The tree is 8-10 feet tall
13 Dec, 2011
Answers
You only show the one leaf, Bob. If there are several opposite leaves on a short stem, in the manner of an ash tree, then it could well be a walnut.
13 Dec, 2011
That was my immediate reaction, Mr B. Seems strange that they've been falling before they are ripe, though. We have thousands of walnut trees in these parts, and the fruit definitely looks right for the role, but the leaf doesn't. That little lop-sided tag at the apex isn't typical. It could just be a different variety, I suppose.
13 Dec, 2011
http://www.onceuponaplate1.com/2008/12/vin-de-noix-you-can-make-your-own.html
I say use them !
14 Dec, 2011
It looks like an evergreen fig to me, though that would be a soft, hollow spot in the center--as it appears--rather than a stone. The stripe leading to the end of the fruit is the remains of a passageway that gnat-sized pollinating wasps use to get to the floral parts inside the fig. They may be dropping unripe because the proper wasp isn't present, and they will only wait so long for some!
14 Dec, 2011
Thank you everyone for your suggestions. I think Pimpernel may have stumbled upon it but I shall keep my fingers crossed and see if they ripen next summer and, if so, what evolves. Watch this space.
15 Dec, 2011
Wallnuts, good.
15 Dec, 2011
;0).. I think the credit due to Bulbaholic.....
15 Dec, 2011
The only trouble is that the tree and its foliage doesn't look anything like a walnut.
15 Dec, 2011
Whoa, where did that tree come from, it wasn't there the last time I looked! OK, now not a walnut :-( and no vin de noix for Bob. Leaf type is wrong.
15 Dec, 2011
No, and where did the pictures showing a single leaf(let) and the whole fruit go? Wrong time of year for brewing vin de noix, anyhow.
15 Dec, 2011
Hmmn! A question can only hold two photo's Gattina..Unclebob would need to do a blog to include all the pictures at once. It would be interesting.
16 Dec, 2011
Aaaahhhh. Honestly P., you know so much.......
16 Dec, 2011
Lychee?
17 Dec, 2011
Or else a Loquat - Eriobotrya. The tree shape and leaves reminds me of when I was at school in the Kenyan Highlands, and we used to sneak over the school fence to nick fruits from a near-by farmer! We were always hungry is my only excuse for the theft! The reason the fruits may be dropping early could be due to lack of water, or too much heat. The kenyan highlands around limuru, at around 5,ooo ft, were on a par with the UK without the ice and snow. The fruits would ripen to orange. The leaves are just the same as loquat trees.
17 Dec, 2011
Hmmm...a loquat would have a pit, or occasionally two or three, and would have a center "passage" leading to a "navel", like the quince relative it is. The leaves would also have pleated look, from the heavy side veins, and woolly backs. I admit that the foliage texture and branch structure still don't look right for a loquat, though.
17 Dec, 2011
I looked at Pimpernel's website again, and the fruits definitely seem to match - any chance of a close up of the leaves?
19 Dec, 2011
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