One of the Duolingo translations of 'koru' is 'piece of jewelry' ... so surely it follows that 'korut' are 'pieces of jewelry'?
The thing is that with English nouns that usually don't have a singular/plural distinction, the "piece of" -premodifier is necessary when it's just one thing, while "pieces of" would be redundant when there are several. Note, for instance, how translating "uutinen" as "a piece of news" makes sense, whereas translating "uutiset" as "pieces of news" would be odd. I reckon that the same applies to jewellery.
As a native English speaker (Australian) I don't recall ever seeing or hearing the word "jewelleries," to me it's not a word. But earlier Duo hints taught us korut means 'jewellery / pieces of jewellery', and here the "ovat" says it's plural. Although the more natural English translation is "The Finnish jewellery is over here," I think it's important to include the alternate "The Finnish pieces of jewellery are over here" as something that matches what we've been taught in Duolingo. It's a slightly different feel to it, but not much. Those "pieces" are perhaps more important and valuable.
Staying as true as possible to the structure of the source language is very rarely a priority in translation. Instead, one of the main priorities is usually a translation that sounds natural in the target language. If you forget the about the Finnish sentence for a moment and just consider whether it sounds more natural to you in English to say "jewellery" or "items of jewellery", then there's your answer. Professional translators generally think in the target language and try to reject negative influence from the source language during the process of producing a translation.
I get what you say about translation, but "items of jewellery" or "pieces of jewellery" are sentence constructs used in preference to "jewellery" that are more common than you might think (saw a lot of instances when having a Google, so I'm satisfied it's not just me). They provide the opportunity to refer to the jewellery as plural. I wondered if maybe this pluralising is a Brit preference ... but could also be a Finn thing if the 'ovat' is anything to go by? Perhaps worth considering as a correct alternative option to plain "jewellery"?
Now that I think about it, there are certainly instances where "pieces of jewellery" ought to be preferred over "jewellery", such as when the noun phrase is modified by a numeral. I suppose Finnish has a bit of preference for pluralising when compared to English. There is at least a handful of words that Finnish pluralises that would be singular in English. Some examples in addition to this one that I can think of off the top of my head are "häät" (wedding), "hautajaiset" (funeral), "juhlat" (party), "kasvot" (face), "varusteet" (equipment), "matkatavarat" (luggage), "hiukset" (hair), and "tikkaat" (ladder).
Although Finnish doesn't have any articles, there are certain ways to convey the meaning of certain articles, such as word order, grammatical case, and certain article-like words. Since the subject is in nominative case and in the beginning of sentence, we're talking about specific pieces of jewellery, and therefore the translation should include the definite article. The subject in the English sentence would be indefinite without the definite article (and since it's an existential clause with an indefinite notional subject, there would be a strong tendency to use an existential there in the beginning of the sentence, as in "There is Finnish jewelry over here"). This indefiniteness would be expressed in Finnish as "Täällä on suomalaisia koruja" or alternatively "Suomalaisia koruja on täällä", which places emphasis on the subject.
In translation, one ought to get into the habit of examining and translating sentences as a whole instead of doing it one piece at a time. The Finnish sentence uses a plural noun, so the verb needs to be plural as well. In the English translation, the translation for "korut", which is "jewelry", is a singulare tantum noun, which is a noun that has no plural form and is only used with singular verbs.