We have the people coming out to the house to measure us for ours next month.
I have tried on a sample of it at a wedding fair and I think it was possibly 4mm, or maybe 5 ...(Don't laugh, have just been sat here with ruler trying to envisage - then dropped engagement ring on floor whilst was trying to measure width of stone and and, and had to crawl under desk to find it!!).
I have very small fingers though (size H) so don't think I'll ever have room for an eternity ring!
Liz
