Few anniversaries are currently based on elements. From a cursory glance, they would appear to include only silver, gold, carbon (diamond), iron, tin, copper, aluminium, platinum and nickel. It would also be possible to stretch the list a little by considering elements in compounds, alloys or mixtures, so salt could become sodium. There are also several incompatible lists.
The entire periodic table should be used instead. If we're talking about wedding anniversaries, that's one heck of a long list even if we only get to uranium, and it would be sensible to stop fairly soon after that due to the highly radioactive and unstable nature of the substances concerned, unless one is to regard that as the end of the marriage through death, which does make some kind of sense if it's done in order of atomic number. Consequently, "anniversaries" should occur more than once a year. It's also unfeasible to do them in order because diamond has an atomic number of six whereas iron, the sixth anniversary, is number twenty-six. Instead of doing that, I suggest beginning with what's currently on a list and choosing, so far as possible, elements significant in the appropriate substances.
Firstly, paper is hydrogen. Tritium EXIT-type sign with "ENTRANCE" on it, since at that stage you will probably still be entranced, plus it's the entrance to a new relationship.
Secondly, cotton is oxygen. Scuba-diving equipment or possibly welding equipment.
Thirdly, leather is nitrogen (in the protein making up leather). Flowers dipped in liquid nitrogen or maybe some high explosive should do it.
Fourthly, silk is sulphur.
Fifthly, there is an issue because every significant element in wood has already been used. I suggest phosphorus. Matches.
Sixthly, iron - the first time something can be left alone.
Seventhly, copper.
Eighthly, salt is sodium.
Ninthly, we have copper again so that needs replacing. I suggest aluminium because the tenth anniversary is sometimes considered aluminium.
Tenthly, we have tin.
Eleventhly, chromium seems to be a fit substitute for steel since stainless steel is high in it.
Twelfthly, selenium can be used to replace silk as it's been used before and selenium is "eka-sulphur".
Thirteenthly, and this is a bit of a stretch, lace can be replaced by chlorine, because there are two types of agate with lace in their names, one of which seems similar to sodalite, which contains chlorine.
Fourteenthly, ivory can be fluorine because of the use of fluorine on teeth.
Fifteenthly, crystal, which probably refers to a kind of glass, can be silicon.
Sixteenthly, there is the start of a hiatus but the suggestion on Wikipedia is to give silver holloware as a gift. In order to find a novel element here, it probably makes sense to use antimony because it's a significant metal in some forms of silver plate.
Seventeenthly, and this too is a stretch, furniture can become manganese because wrought iron is sometimes used to make furniture and that sometimes contains that element.
Eighteenthly, I suggest replacing the white porcelain suggested on the same list with titanium, since it's in titanium dioxide, a startlingly white compound used to provide whiteness.
Nineteenthly, thank you for reading this far and I suggest arsenic, a component of some bronzes. This would provide the opportunity for a suicide pact or murder, which would be appropriate for some marriages.
Twentiethly, platinum is one suggestion so just leave it at that.
Twenty-firstly, nickel.
Twenty-secondly, well, copper's already been done but zinc is next to it and pretty common, so why not just use that?
Twenty-thirdly, following that principle cadmium is the next one along from silver, this one being silver plate.
Twenty-fourthly, the next one is silver so let's use the one before silver, which is palladium.
Twenty-fifthly, we have silver itself of course.
Twenty-sixthly, we have another gap, so I'm just going to start work my way across the post-transition row, having already used cadmium, and say indium.
Twenty-seventhly, tellurium, tin and antimony having already been used.
Twenty-eighthly, iodine.
Twenty-ninthly, xenon. I quite like this idea because it's expensive.
Thirtiethly, calcium, since it's traditionally pearl, which is calcium carbonate.
Thirty-firstly, there's another gap, so I shall now start working across the first row of the transition metals and say scandium.
Thirty-secondly, titanium has already been used so vanadium is the next one.
Thirty-thirdly, cobalt is the next unused element.
Thirty-fourthly, rubidium is the one before strontium.
Thirty-fifthly, there is traditionally coral or jade. I choose to go for strontium because it burns with a coral flame.
Thirty-sixthly, here we are again with a bit of space so I'm going to say yttrium.
Thirty-seventhly, I finally get to use zirconium, which I considered for crystal because of the use of cubic zirconia to replace diamond, but what the heck, I'll just stick it here.
Thirty-eighthly, niobium seems about right here. Who, after all, hasn't felt that the number thirty-eight has a kind of niobic quality from time to time?
Thirty-ninthly, molybdenum seems to be the obvious choice.
Fortiethly, we would have ruby except that that is red due to chromium, so instead I am going to suggest mercury because of its presence in the red mineral cinnabar. This is really contrived of course.
For the forty-first to the forty-ninth, the nine elements before gold, skipping platinum, seem appropriate, so those are thulium, ytterbium, lutetium, hafnium, tantalum, tungsten, rhenium, osmium and iridium. I've been neglecting the rare earths, so it's quite a relief to use them at last, if only three of them.
Following on from gold but skipping mercury, fifty-one and fifty-two can be thallium and lead. It's a bit peculiar that lead isn't already in there somewhere in fact.
Since the fifty-fifth is emerald, which contains beryllium, meaning it can be that, numbers fifty-three and fifty- four can be helium and lithium.
The fifty-sixth can be neon, because it's nearby and gets me for once to coordinate the atomic numbers with the next two, lanthanum the fifty-seventh and cerium the fifty-eighth.
Approaching diamond, which is carbon, the fifty-ninth can be boron.
The sixtieth is carbon, which could save a lot of money since it could just be a piece of graphite or coal instead of a real diamond.
After that it all seems to get a bit vague, so I don't know about the rest. However, rather than just continuing to fill them in above sixty, I would prefer only to go up to Americium at number ninety-five, leaving another thirty- five to be peppered through the sixty-year period regularly, every year and a half or so. I am particularly keen on uranium - uranium glass gifts seem quite groovy - plutonium - give your partner a nuclear warhead for that anniversary to use in arguments, or maybe a nuclear power station - and americium - smoke detector.-- nineteenthly, Jan 16 2016 Wow, this took a lot of work and thought. I like. +-- blissmiss, Jan 16 2016 Thanks, but probably not enough thought. It's a bit arbitrary.-- nineteenthly, Jan 16 2016 This would only make sense if you stick to atomic number order.-- MaxwellBuchanan, Jan 16 2016 That would make diamond the sixth anniversary. I would like to come up with a function which puts the ones which already exist in the right places while placing the others in some kind of logical order.-- nineteenthly, Jan 17 2016 It could make graphite the sixth anniversary. Nevertheless, adhering to atomic number is a good idea because the gifts would become increasingly exotic as the couple ages.
On the other hand, you could arrange the metals by decreasing reactivity. The cheaper, common metals are more reactive than the precious metals.-- Cuit_au_Four, Jan 17 2016 Say old chap, what did you get your wife for your 33rd Anniversary? Why I bought a knife and gave her arse a nick. It's traditional you know.-- AusCan531, Jan 17 2016 I like the idea of the stuff getting progressively deadlier as the years go by.-- Ander, Apr 12 2016 Did you post this idea solely so you could legitimately use the word "Nineteenthly" in a real sentence? Also, the noble gases seem under-represented, otherwise an excellent idea.-- hippo, Apr 12 2016 The 85th anniversary is going to be a little tough to solve. Possibly beyond the 96yo couple trying to solve it...-- bs0u0155, Apr 12 2016 They could give each other the entire planet. It's sure to have a little astatine in it somewhere. Less than a gramme, but some.-- nineteenthly, Apr 12 2016 random, halfbakery