Which is a pity, because they involve one decision that is genuinely irreversible and several that are easy to get wrong.

Here is what these rings actually are and what to know before buying one.

The terminology which is looser than it sounds

An eternity band is a ring with stones set all the way around the band. Full circle, no plain section.

A half eternity band, sometimes called a partial eternity, has stones across the top half or so, with plain metal underneath where the finger bends.

An anniversary ring is not a technical term at all. It simply means a ring given to mark a milestone, and it can be an eternity band, a plain band, a ring with a single stone, or anything else. The name describes the occasion, not the object.

Nobody polices any of this, and jewellers use the terms loosely, so it is worth asking what is actually meant rather than assuming.

The one decision you cannot undo

This is the single most important thing in this article, and it is routinely not mentioned at the point of sale.

A full eternity band cannot be resized.

There is no plain section of metal to cut, add to or remove. The stones run continuously, and altering the circumference means remaking the ring. Some can be adjusted a fraction by a skilled jeweller, and some cannot be adjusted at all.

Fingers change over a life. They change in pregnancy, in heat, with weight, with age and with arthritis. A ring bought at forty may not fit at sixty, and a ring that no longer fits is a ring in a drawer.

A half eternity band resizes normally, because the plain lower section can be cut and adjusted like any band.

That is not a reason to avoid full eternity bands. It is a reason to make the decision knowingly. Plenty of people buy one understanding they may eventually need it remade, and that is a fair trade for a ring that looks the same from every angle.

Half eternity has a second practical advantage that people rarely consider: plain metal underneath is far more comfortable against an adjacent ring, and it does not dig in when you grip something.

How the stones are held

The setting style changes the look, the durability and the price considerably.

Channel setting runs the stones between two continuous walls of metal. Very secure, smooth on the outside, nothing to catch. The most practical option for daily wear and the one most likely to survive decades without intervention.

Shared claw or shared prong uses single claws sitting between adjacent stones, so each claw holds two. This lets far more light into each stone, so the ring is brighter, and it means more exposed metal that can wear or catch.

Bezel setting surrounds each stone individually with a rim of metal. The most secure of all, with a clean modern look, and fewer stones fit into the same circumference because of the metal between them.

Pave and micro-pave set many small stones held by tiny beads of metal. Beautiful, glittering and the highest maintenance of the four. Small stones do occasionally come out and need replacing.

If you want one ring that will still be fine in thirty years without much thought, channel or bezel. If you want maximum sparkle and accept some upkeep, shared claw or pave.

What drives the price

Not what people expect.

On a ring with a single stone, the stone is most of the cost. On an eternity band it frequently is not, because you are buying a great deal of labour.

Each stone has to be individually seated and set. A band with thirty stones is thirty setting operations, performed under magnification, each of which has to be perfect because one that is slightly off is visible against its neighbours.

That means two things. First, an eternity band is a good test of a workshop, because uneven stones and inconsistent spacing show immediately. Second, the price difference between a cheap one and a good one is mostly bench time rather than materials, and it is visible.

The stones themselves are usually small, which keeps per-stone cost low. Total carat weight is the number quoted, and it is worth asking how it is distributed, since twenty larger stones and forty smaller ones can share the same total and look quite different.

Matching it to rings you already own

The part that goes wrong most often.

Match the metal hardness, not just the colour. If a softer band sits against a harder ring, the softer one wears on the contact surface over decades. Platinum against platinum, or 18 carat gold against 18 carat gold, wear evenly. Mixing metals for a deliberate two-tone look is entirely fine, but match the hardness where you can.

Check how it sits against the engagement ring. A straight eternity band against a ring with a raised or curved setting can leave a visible wedge of gap. A contoured band, shaped to nest against the existing ring, solves it and is a custom job.

Think about stacking order. Many people wear the eternity band on the outside, where it takes more contact, or between the engagement and wedding rings, where it is protected. Both are common and the choice affects wear.

Consider width carefully. Three rings on one finger is a different proposition from one, and a millimetre or two of extra width changes how the stack feels substantially.

What to buy for what milestone

There is no rule and anyone who tells you otherwise is selling something. That said, a few patterns are common.

Early anniversaries tend toward a plain or narrow stone-set band to sit alongside the existing rings.

Ten years is when eternity bands appear most often.

Significant milestones, twenty-five and beyond, are frequently when people reset or redesign rather than add. A worn original setting is remade, sometimes with the addition of new stones, which produces a ring that carries the history and suits the present.

Births often prompt something with coloured stones marking each child, which is a lovely use of gemstones that are otherwise hard to place.

And a substantial proportion of these rings are bought by the wearer for themselves, which is entirely normal and increasingly common.

Coloured stones as an alternative

Worth considering, because a second diamond band can feel like a repeat of what is already there.

Sapphire is the most practical coloured option. Hard-wearing, available in blues, teals, greens, peaches and greys, and Australian parti sapphires with zones of different colour within one stone are genuinely distinctive.

Ruby and spinel are similarly durable and give strong reds and pinks.

Avoid for daily wear: emerald and opal are beautiful and comparatively fragile, and emeralds are typically oil-treated in ways that degrade. In a ring worn constantly against other rings, they are a maintenance commitment.

Sizing since this matters more here

Because full eternity bands cannot be adjusted, sizing deserves more care than usual.

Size in the afternoon, when fingers are at their larger end. A ring that fits at your largest is wearable at your smallest. The reverse gets stuck.

Size the actual profile. A wide band sits differently from a narrow one and frequently needs a slightly larger size.

Size on the finger it will actually live on, and in the position it will sit in the stack, since a ring between two others fits differently from one worn alone.

If your size fluctuates seasonally, this is an argument for half eternity.

Buying one well

A few questions that sort the market quickly.

Are the stones matched for colour and size? On a band of many small stones, variation between them is the most common quality failure and the easiest to see once pointed out.

Is the spacing even all the way around? Look at it from directly above under good light.

How are the stones held, and what happens if one comes out? Ask specifically whether replacements are matched and what it costs.

Can this be resized, and by how much?

Where was it made? A business that does its own setting on site can also repair it on site for the next forty years, which on a ring with this many stones is worth more than it sounds.

Independent Australian workshops that design and set in house, Stelios Jewellers among them, tend to be the places where eternity bands are made to match an existing ring rather than bought off a shelf and hoped for. Given how often the fit against an existing engagement ring is the problem, that matters.

Looking after it

Bands with many small stones need slightly more attention than a solitaire.

Have it checked annually. Small stones loosen before they are lost, and a check catches it.

Clean it regularly. Warm water, a drop of dishwashing liquid and a soft toothbrush. Many small stones mean many small crevices, and an eternity band that looks dull is usually just dirty.

Be careful with ultrasonic cleaners if the stones are pave set, since vibration can loosen stones already sitting on worn beads.

Insure it properly. A valuation is a different document from a grading report, and most home and contents policies cap jewellery per item unless it is listed separately.

Resetting instead of adding

Worth raising because it is frequently the better answer and it is rarely offered.

By a significant anniversary, an original engagement ring has usually had twenty or thirty years of wear. The band has thinned where it rubs against the finger, the claws have worn down, and the setting may be tired even where the stone is perfect.

At that point, adding a new band beside it is one option. Remaking it is another, and it often produces something people love more.

The stone comes out of the worn mount and goes into a new setting, sometimes with additional stones added to mark the years. The material is the same, the meaning is unchanged, and what you end up with is a ring built for the hand you have now rather than the one you had at twenty-five.

This is also frequently the best value work a jeweller does, because the most expensive component is already yours. You are paying for metal and labour rather than for a stone.

If you are considering it, keep the original setting rather than trading it in. It can sit in a box, or be reworked into a pendant, and the idea that remaking destroys the original is what stops most people from doing something they would be pleased with.

A note on buying it as a surprise

Anniversary rings are given as surprises far more often than engagement rings are, and the sizing problem is the same one with a useful advantage: you already have access to rings she wears.

Borrow one from the correct finger and take it to a jeweller. That is by far the most accurate method, and unlike an engagement ring there is usually no secrecy problem about the ring being briefly missing.

What is harder is style. Someone who has worn the same rings for twenty years has firm views, and a band that does not suit the existing stack will not get worn. If in any doubt, a jeweller will hold a design or produce a resin prototype, and plenty of couples do the surprise on the occasion and the choosing afterwards.

The short version

Half eternity if you want a ring you can resize for the rest of your life. Full eternity if you want the look and accept the constraint.

Channel or bezel setting for durability, shared claw or pave for brightness and more upkeep.

Match the hardness of adjacent rings, check how it sits against what you already wear, and size in the afternoon.

And remember that most of what you are paying for is setting labour, which means the difference between a good one and a cheap one is genuinely visible from a metre away.

This article is general information only. Confirm specifications, resizing limits and insurance requirements with your jeweller and insurer.