Nice work on the site. The products look lovely.

If I was looking for something specific, I'd probably google "Amethyst earrings" or "silver rings". If I wanted something more general I'd probably look for "jewellery gift ideas".

One piece of advice I would give is that the "splash" pages for each of the sections just doesn't exist as far as the search engines are concerned. There's no alt tag, even, on the image. And as a consumer, I find it quite confusing. The images are lovely and very stylish, but I'm kind of "well where are the products then" - especially on the necklace splash page, where the red text is very hard to read. I'd consider using that splash page to show one or two bestsellers from those categories, and then add some keyword-rich text so the page. (So make the splash image a background to a table cell.) On some of them you might need to add a semi-opaque panel so the text is easily readable.

Also for optimising, not sure what commerce solution you're using, but oscommerce and ZenCart both have add-ons (free) which will change the urls of the products to something more meanginful. So instead of
http://www.oishii-jewellery.co.uk/proddetail.asp?prod=CHN101
you get
http://www.oishii-jewellery.co.uk/proddetail.asp?prod=silver-ice-necklace
which gets picked up by the engines and adds to the google rank.

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"you are terrifying
and strange and beautiful
something not everyone knows how to love." - Warsan Shire