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All diamonds supplied by Costco.co.uk are as follows: Clarity to be VS2 or better, colour to be I or better as outlined above on the Industry Grading Guide.

Diamond Quality
Internally Flawless Very very slightly included Very slightly Included Slightly included Imperfect
IF VVS1 VVS2 VS1 VS2 SI-1 SI-2 I1 I2 I3
Colour scale in order of best to worst colour
Colourless Near Colourless Slightly Tinted Very light Yellow Light yellow to fancy Yellow


Diamond Education: The 4 C's Diamonds: Carat, Cut, Colour & Clarity


Carat (Weight & Size)

  • The weight or size of a diamond is measured in carats (ct.).
  • One carat = 100 points.
  • Half carat = 50 points.
  • Quarter carat = 25 points.
  • The heavier and larger the diamond, the more rare it usually is.

Cut (A good cut brings out more sparkle)

  • Cut is not the shape of a diamond - it only refers to the arrangement of a diamonds facets.
  • When cut to good proportions, the diamond is better able to handle light, and creates more brilliance and more sparkle.
  • A well-cut diamond will always look brilliant, no matter what setting it's in.

Colour (Colourless is extremely rare)

  • Diamonds come in a range of natural colours, from totally colourless to shades of yellow or brown.
  • A completely colourless diamond is most rare.
  • Colour is a personal choice - choose your diamond based on how its subtle shade appeals to you.

Clarity (Perfectly clear is a rare find)

  • Most diamonds contain very tiny inclusions that make them unique.
  • The fewer and smaller the inclusions, the rarer the diamond.
  • To determine a diamond's clarity, it is examined under 10 x magnification by a trained eye.

Diamond Shapes

Diamond Characteristics

General Information


Costco makes purchasing the highest quality diamonds easy by offering only the industry's highest quality diamonds at the lowest possible price. Our long-standing relationship with our suppliers guarantees we get the best selection at the best value. Costco's experienced buying team works with our suppliers evaluating and hand-selecting each diamond for its quality and brilliance. Our gem laboratory or an independant laboratory then inspects every diamond to ensure it meets the strictest quality standards in the industry in carat, cut, colour and clarity. You can be confident your purchase is as special to us as it is to you.

These items come with a Summation Of Appraisal certificate from the International Gemological Institute (IGI) or a Safeguard certificate. This is to certify that the item described above is genuine, and has been independently examined in the laboratories of IGI or Safeguard .

Also included with these diamonds is a GIA (Gemological Institute of America), Gem Trade Laboratory, Diamond Grading Report. Gem Trade Laboratory is a division of GIA Enterprises, Inc., a wholly owned subsidiary of the nonprofit Gemological Institute of America, Inc. This describes the characteristics of the diamond at the time of the examination based upon 10x magnification.

Frequently asked questions about Jewellery

Can I have the item sent to a costco.co.uk Wholesale location for preview and/or purchase?

No. All items must be purchased through the website and are shipped directly to you. Item selection on costco.co.uk generally differs from the warehouse selection. If you are not completely satisfied, your purchase is fully refundable through costco.co.uk.

Can I order a piece of jewellery in a different metal/different ring size or have my ring sized prior to shipping?

No. We are not able to customise items that are offered on costco.co.uk. All rings are in a standard size N. When a ring has the capability of being sized, we recommend you take the ring to a reputable jeweller in your area as we do not offer this service.

Does costco.co.uk sell loose diamonds?

No. All diamonds sold at costco.co.uk are sold as finished pieces of jewellery.

Can I buy a ring without a centre diamond?

No. costco.co.uk does not sell semi-mounted jewellery.

Can I have a bigger or smaller diamond?

No. Items on the website come with the specifications listed on the site. We are not able to make any modifications or customise items on the website.

Can I expedite shipment of an order?

The estimated delivery time for receipt of an item is listed on the website. To ensure our quality is never compromised and our orders are promptly fulfilled, we are not able to expedite shipments.

Why do I sometimes only see a millimetre (mm) size on gemstones and not an exact carat weight?

Coloured stone cutting is not an exact science. When listing a piece of coloured stone jewellery we guarantee a minimum millimitre size. Coloured stones are often bought and sold according to a calibrated millimitre size because carat weights can vary widely.

I saw an item on the website and it is no longer there, can I special order it?

No. An item is removed from the website when the available inventory has been sold out. When we secure additional inventory, the item will be put back on the website and made available for purchase, if possible.

Who do I contact if I need additional information on a product?

For questions regarding jewellery found on costco.co.uk, please email your question(s) to onlinediamonds@costco.co.uk.

What is costco.co.uk's stance on conflict diamonds?

costco.co.uk shares the concern of the global community in abolishing conflict diamonds, and is committed to procuring goods of the utmost integrity. We are proud to sell beautiful, high quality diamonds that are obtained solely through closely monitored legal channels set forth by the United Nations' Kimberley Process.

The Kimberley Process, adopted into UK legislation, requires manufacturers to comply with a lengthy procedure which closely tracks, ships and receives each diamond, documenting each step so as to monitor the stones' origin. Diamonds that we offer for sale are procured from legitimate sources not involved in funding conflict and comply with the United Nations resolutions set forth by the Kimberly Process procedure.

IGI®/Safeguard® vs. GIA®/AnchorCert®, What's the difference?

International Gemological Institute (IGI®)/Safeguard®, issues Summation of Appraisal, which is an appraisal of a piece of jewellery suitable for insurance purposes. Gemological Institute of America (GIA®)/AnchorCert®, issues Diamond Grading Reports. This is a complete technical description of the diamond as it is loose, prior to being mounted in the setting. GIA®/AnchorCert® do not assign any monetary value to the diamond, just specific measurements and quality grades, identifying that particular diamond vs. any other. If a diamond has a Diamond Grading Report from GIA®/AnchorCert®, the IGI® /Safeguard® will reference that report in the appraisal.

Why do some jewellery items have an IGI® and GIA® PDF (Portable Document Format) file to view and some do not?

Our unique (only one available) jewellery items include a PDF of the actual IGI® appraisal. On items not designated "Only One Available", we often have multiple inventory of the same item. Therefore, we do not show individual PDFs, even though the item may include its own IGI® appraisal.

Can I get the IGI®/Safeguard® and/or GIA®/AnchorCert® certification sent to me prior to ordering the item?

Each item is fulfilled from the inventory available at the time the order is pulled for shipment. When the order is pulled for shipment the appropriate IGI®/Safeguard® and/or GIA®/AnchorCert® certification will be pulled and sent with the item. To ensure our quality is never compromised and our orders are promptly fulfilled, we are not able to hand select or specify diamonds for fulfillment of individual orders.

What does "VS-2 clarity, I colour, or better" mean?

costco.co.uk guarantees diamond clarity to be VS-2 (Very Slightly Included) and colour to be I (Near Colourless) or better. Gemologists check every diamond to ensure our minimum quality is met. This does not apply to watches.

What comes with an appraisal?

Jewellery items containing a 1.00 carat diamond or larger include an IGI® or AnchorCert® appraisal. Jewellery items containing a 1.25 carat diamond or larger include a GIA® Diamond Grading Report, in addition to the IGI® or AnchorCert® appraisal. One-of-a-kind diamond items will include both an IGI® or AnchorCert® appraisal and GIA® Diamond Grading Report regardless of carat weight.

Care & maintenance for your diamond jewellery

Diamonds are one of the hardest substances on earth, but they are still vulnerable to chips, dirt, dust, perspiration, makeup, and natural skin oils, as well as household chemicals, like chlorine and hairspray. With the proper care, cleaning, and storage, you can keep it looking as beautiful as the day you first received it. Always take extra care when wearing your jewellery in the kitchen or the bathroom, especially while cleaning. If you knock your diamond on a hard surface, the stone can become chipped or loose in its setting and you could risk losing your diamond. We recommend that you remove your diamond while cleaning, gardening, swimming and intense exercising.


We recommend having your diamond professionally cleaned and inspected once every six months by a qualified jeweller. This should include checking the mounting for loose stones, prong repair and inspecting the general condition of your jewellery.


To avoid dirty build-up from cosmetics, natural oils, dirt, soap, etc., periodic cleaning is suggested. This will keep the surface of your diamond brilliant and shiny. You can use professional or home-made cleaner. Diamond Cleaning Solution: 1 part ammonia to 6 parts water; gently hand-scrub with a soft-bristle brush.


Accidents do happen and we strongly recommend that you insure your diamond against loss, theft, and damage.