The simanim that were used by Abayye in the Gemara are, for the most part, easy to identify and well accepted. Kara is squash, Karti is leeks, Silki is beets (probably the healthy green leaves), and Tamri is dates. The identity of the rubia has proved more elusive, resulting in a variety of contending customs, and providing us with a glimpse into the halachic, linguistic, and botanical wisdom evinced by our most illustrious poskim in deciding between them.
Most people today probably associate the rubia with black-eyed beans, the minhag of many Sepharadim and Yerushalmi Ashkenazim. Much less known is a mighty contender to wear the rubia crown, championed by none other than Rashi himself, codified in the Shulchan Aruch, and consumed to this day as rubia by the entire Yemenite Jewish community. Our contender goes by many names. Rashi refers to him in Old French as fenugri, aka fenugreek in English. He uses the term in describing the Hebrew tiltan, a three-leafed plant mentioned in the Mishna, whose bean-like kernels are mashed and prepared into a healthy and, in this writer's opinion, a delicious spread, enjoyed daily as a part of Yemenite and Modern Israeli cuisine under its popular Arabic name: hilba. Yes, that mysterious green goo at the falafel stand may very well be none other than the original rubia, our beloved siman of plenitude.
However, our reigning champion rubia, the black-eyed bean, boasts an even stronger pedigree. Rav Hai Gaon identifies the rubia as the Arabic lubia, aka, or closely related to, the Egyptian Bean. Rambam and Kaftor V'ferach identify sh'uis of the Mishna as the same lubia. This should not be confused with the sh'uis of Modern Hebrew, broad beans, generally white, that originally hail from South America, and hence, were not consumed by Hazal and the Rishonim. Since the rubia/ lubia is after all a siman tov, it stands to reason that the selfsame sh'uis of the Mishna would be a most beloved food. Indeed, we do find that the Amora Rebi Yona in the first perek of the Talmud Yerushalmi Kilayim explains the name sh'uis as a derash: It is called sh'uis because it mesha'a'sha'as the heart and mehaleches the intestines. This Gemara is understood simply according to the Pnei Moshe, the pre-eminent commentator on the Yerushalmi and possible childhood rebbe of the Gra (see here), as stating that sh'uis is a delightful food, which stimulates the digestive process. Interestingly, a diametrically opposite interpretation is offered by the rishonim, Rash and Rivmatz. Mesha'a'sha'as is understood as to paste up, an alternate meaning of the root shin ayin ayin, which is also found in Tanach. They explain that sh'uis causes timtum haLev, it closes the mind - and causes diarrhea! If so, how could it possibly be a siman tov? This contradiction is resolved upon realizing that they did not identify the sh'uis as lubia, like Rambam, but rather as a different legume, vetch, the Aruch's identification. The loathsome vetch is known in the Yerushalmi as animal fodder, not really fit for human consumption (as it is full of toxins). One may even do business with it during sh'vi'is. It was common in Europe, home of the Rivmatz and the Rash, and generally only consumed by humans in times of famine. The wonderful rubia, on the other hand, was not available at all in Europe as it only grows in hot climates. This is even suggested by its name.
One would thinkthat the Arabic name lubia is a corruption of the original name rubia, so important for our siman, but Rav Matzliah, disciple of Rav Hai Gaon, states that lubia is the original name since it is native to thecountry of Libya, and hence, a close neighbor to the Egyptian bean! (S. - Apart for the fact that it is equally plausible for rubia to be a corruption of lubia, I would just add that /r/ and /l/ are both liquid consonants, and switch quite easily - witness the notorious difficulty Japanese and other East Asians have pronouncing them as Westerners do (link). In addition, if one considers a resh pronounced in traditional fashion, not equivalent to an English R, then the close connection with /l/ is even more apparent to the ear. To put it more simply, in my opinion neither need be a corruption of the other. They could both be perfectly valid spellings for the same word.)
Black-eyed beans on New Year's day are prized by many non-jews of the American Southeast, who consume then in a festive meal on January 1 to bring wealth in the following year! Unlike us, though, most of them eat the beans with pork. Nevertheless, the regnant theory in explaining the genesis of this custom is that it was first learned by African slaves from Sepharadi Jews who arrived in the region almost 300 years ago with their black-eyed beans custom.
Other lesser contenders for the rubia title have been the three-leafed clover, aka the Modern Hebrew tiltan, the national symbol of Ireland, but rejected outright by Chida; and among North African Sephardi communities, fennel, sesame, and cabbage (apparently because kruv sounds “like” rubia) have been consumed. It appears that these communities once tried to adopt the custom without a mesora from Spain or Bavel. By now, they too have “increased their merits” by joining klal yisroel in marking their siman tov with black-eyed beans – or hilba!
(S. - I also wanted to add that plants of the Madder family are known as Rubia, from the Latin, because they produce red dye. They appear to produce edible berries, are native to the relevant regions in the Talmudic period, and although I have no idea if anyone ever suggested this, in my opinion they should also be investigated as a candidate.)