A black etrog is invalid, because this is not the normal color of an etrog (SA 648:17). An etrog that is dark green is invalid because it is immature. But if it is clear that it will turn yellow under the right circumstances (e.g., if left to ripen with apples), even when it is still dark green, it is kosher (SA 648:21). An etrog that has turned orange is kosher (Mor U-ketzi’a 648).
For an etrog to be kosher, it must be fit to eat. Therefore, an etrog is invalid if it is orla or if it has not had teruma and ma’aser taken from it (MT, Laws of Shofar, Sukka, and Lulav 8:2).
An etrog is invalid if it is smaller than a keveitza (c. 50 cc), because it is immature. However, if it is a keveitza, then even though it is still unripe, it is kosher. There is no upward limit on size. Even if carrying an etrog requires both hands, it is kosher (SA 648:22). Some are stringent and require the etrog to be at least the volume of two eggs(100 cc). Although we do not follow this opinion, but rather rule that an etrog of 50 cc is kosher (Peninei Halakha: Berakhot 10 n. 11), le-khatḥila it is preferable to beautify the mitzva, and part of this is for the etrog to be normal-sized and not small.
A dried-up etrog – one that does not discharge any liquid, so that if one pierces it all the way through with a threaded needle, the thread will remain dry (SA 648:1) – is invalid because it lacks hadar (Sukka 31a and 34b). Any etrog left over from the previous year is assumed to be dried out (Rema ad loc.). However, if it was kept carefully in a refrigerator or a sealed bag, even after a year, it may retain some moisture and thus be kosher (Bikurei Yaakov ad loc. 4; SHT ad loc. 8).
An etrog is invalid if its shape is totally different from that of a regular etrog – for example, if it is round like a ball or was grown in a square container. However, an etrog that has two tops but is fused at the bottom, like conjoined twins, is kosher, as it is not totally different from the standard shape (SA 648:18-20).
