It occurred to me that there’s one concept I have never seen anyone else define or use, although I’ve been working with it in my own research for a while now: that of an inheritance phoneme.
This is in effect the polar opposite of the well-known case of the loanword phoneme. As the audience of this blog probably mostly knows, a loanword phoneme refers to a sound that is absent from the native lexicon of a language, but occurs in one or more of its contact languages, and has been taken on from there into the language itself. Clear examples include /b g f ʃ/ in modern Finnish.
But sometimes, we can by contrast find in a language a phoneme that is absent from its contact languages, and is only found in the native-enough lexicon.  In Finnish a recent example might be the labial opening diphthongs /uo/, /yö/. Although found as reflexes of earlier *oo, *öö even in some not especially old loanwords from e.g. Swedish (including tuoli ‘chair’, kyöpeli ‘kobold’; yet more recently also fluori ‘fluorine’), they appear to have within the last about 200 years become a “closed class” that, for now, is no longer acquiring new members.  Of course, this is not “closed” in the same sense as a morphological word class might be — the diphthongs remain entirely possible in new ideophones and onomatopoeia (blyögh ‘barf!’), blends (Suomalia ‘an area in Finland with a relatively large current or predicted Somali population’), and derivatives based on pre-existing roots.
Better examples can probably be found, from languages having some more strongly marked phonemes. For example, I’d expect Czech ř or German pf to be not very common in current loanwords, and to have been so for a good while; or the nasal vowels in French to be absent from most modern loanwords, with the exception of those from Portuguese or sub-Saharan African languages.
Even then, this concept seems less clearly defined than the loanword phoneme. While a loanword phoneme is established by its one-time inadmissibility in the language altogether, there is nothing in a language’s internal structure at any given time that could prevent a given phoneme from appearing in loans. This situation can only be an incidental fact about its contact languages — and if the contact situation changes, anything’s possible again. (Put a Czech speaker community in regular contact with speakers of Toda, and I for one would bet that ř would then start regularly turning up in some loanwords.) A phoneme could also be only “partially inherited”, in being found in some loan strata but not in others — as I hypothesized to be the case with French nasal vowels.
On the other hand, what is interesting here is that while words containing loanword phonemes allow setting up a terminus post quem for their acquisition into the language (if we know that Finnish circa 1600 had no /ʃ/, then all modern Finnish words with the consonant must be more recent, even if their etymology were unknown) — inheritance phonemes may allow establishing a terminus ante quem. This seems like a fairly powerful tool; usually we can backdate a word only by the comparative method, and even then not watertightly either. But, given a word like Fi. tuoksua ‘to smell’ (of unknown origin, not attested before the end of the 17th century, and in contrast to the more widespread native Finnic synonym haista), we can regardless consider it probable from its diphthong that this is not an especially young word, perhaps dating at least to the Middle Ages. Given an absense of known loan etymologies from any obvious candidates for a loangiver (Swedish, Russian etc.) would furthermore suggest that we can with slightly lower confidence add a couple of centuries more yet. 
We can also define similar concepts such as loan cluster and inheritance cluster. The former, although to my knowledge never explicitly named, is again a known phenomenon. Finnish continues to work as an example: while Modern Finnish clearly allows e.g. word-initial consonant clusters, it is not too hard to find phonological analyses that dismiss them as non-native and proceed to posit a “basic” syllable structure (C)V(V/C)(C). Jorma Koivulehto has also made good use of this approach in research of early loanwords, having e.g. shown that all Finnic word roots with the medial cluster *-rt- are ultimately Indo-European loans, and not of Uralic inheritance.  (This, however, is not to be confused with the occurrence of *rt in word stems, where it can well result from inherited *r + a suffix such as causative *-ta-; as in Fi. vieri ‘side’ → vier-tä- ‘to be or go beside smth.’)
It seems similarly possible to consider e.g. Finnish tk for the most part an inheritance cluster that indicates relatively native vocabulary. No examples of this cluster in old loans are known; and given that already in Late Proto-Indo-European, the inherited “thorn” clusters of dental + velar were metathesized or otherwise reduced, it seems likely that none will be found anytime soon either, at least not from an Indo-European direction. (Much newer examples can be found though, e.g. Atkinsin dieetti, votka; and in far-northern dialects, e.g. vietka ‘adze’, from Sami.)
I could explore various further examples here, but for now, this post should do for a point of reference for later use.
 “Nativeness” is a relative concept, of course, not an absolute one. E.g. Finnish kauppa ‘store’ can be considered a “native” counterpart of the more recent loans puoti (← Swedish), lafka (← Russian), basaari (ultimately ← Persian) etc., but ultimately it is a Germanic loanword as well. Similarly, even words reconstructible back to Proto-Uralic can in principle be loans at some deeper time-level yet (e.g. we can suspect on semantic grounds that pata < *pata ‘pot’ might be one).
 The illabial opening diphthong /ie/ remains possible in loans, e.g. fiesta, siesta, DJ Tiësto.
 For some speculation though, something could be perhaps made of the similarity to Swedish doft, German Duft ‘smell’. If these could be analyzed as earlier *duf-t-, perhaps in turn some kind of a labial-stop extension of PIE *dʰewh₂- ‘to smoke’ (PG *dup-?? Svensk Etymologisk Ordbok connects here also Greek τυφος ‘smoke’), then we might be able to assume that the Finnish word derives from pseudo-PF *tupa/*tupo ‘smell’ → *tuβa-ks-u-/*tuβo-ks-u- ‘to put out smell’ > *tu.aksu-/*tu.oksu-, with a similar late contracted diphthong as in words like siellä < *si.ällä < *siɣällä < *sigä-llä ‘there’, or haukka < havukka (attested dialectally) < *haβukka < *habukka ‘hawk’.
 See in particular: Koivulehto, Jorma (1979): Baltisches und Germanisches im Finnischen: die. finn. Stämme auf -rte und die finn. Sequenz VrtV. In: Schiefer, Erhard F. (ed.), Explanationes und tractationes Fenno-Ugricae in honorem Hans Fromm, pp. 129–164. München.