Find out what her current rings are made of before anything else. A white metal set with a yellow gold anniversary band can look great, but only if the contrast is a choice rather than an accident. Then check the width. A new band that matches her wedding band keeps the finger tidy; one that is clearly wider or thinner should look different on purpose.
Her habits count for more than tradition. If she wears the diamond ring every day but often leaves the wedding band in a dish, a new ring that works on its own, or that takes the old band's place, will get more wear.
There is no fixed rule. Some wear the anniversary band on the same finger, above the engagement ring or below the wedding band, so the original pair stays side by side. Others move it to the right hand, which stops the left from getting crowded and gives the gift its own spot. Three stone designs suit that right hand role well because they are often read as past, present and future; browse three stone rings if the idea appeals.
Borrow a ring she wears on the same finger, then follow Rare Carat's step by step ring size guide to turn it into a size before you slip it back. If the gift is going on her right hand, borrow a right hand ring, because fingers on the two hands are often slightly different sizes. A sister or close friend may already know the answer.
With diamonds all the way around, the size has to be right first time, since a full eternity ring is hard or impossible to resize. When in doubt, choose a half eternity style or a band with plain metal underneath, which a jeweler can adjust later.
An anniversary is a natural moment to add the sparkle the original set may not have. A row of diamonds across the top, or a full circle, sits comfortably beside most engagement rings; the eternity rings and bands collection is a good place to compare. Diamonds grown in a lab stretch the budget further, as they cost far less than mined stones of the same size and grade; see how they look set in lab grown diamond rings.
If you would rather spread the cost, Rare Carat offers pay over time plans through Affirm, Klarna, Afterpay, PayPal Pay Later and Katapult.