Blue topaz is the gemstone designated for the fourth wedding anniversary in the modern anniversary gift list promoted by Jewelers of America, the principal trade body responsible for standardising anniversary gem associations in the United States. The designation has been widely adopted by jewellers and gift-givers internationally, supplanting the older traditional lists that assigned materials such as linen or silk to this milestone. Blue topaz's combination of excellent clarity, strong blue saturation, and broad commercial availability makes it a practical and visually compelling choice for commemorative jewellery marking a marriage's entry into its fifth year.

The Gemstone

Topaz is an aluminium fluoro-silicate mineral, with the chemical formula Al₂SiO₄(F,OH)₂, belonging to the orthorhombic crystal system. Pure topaz is colourless; the blue hues associated with the fourth anniversary designation are almost universally the product of treatment rather than natural colour. Natural blue topaz does occur — notably from localities in Brazil, Sri Lanka, and Nigeria — but it is typically pale and commands little commercial interest in its untreated state.

Treatment and Colour Varieties

The vivid blues seen in contemporary jewellery are achieved through a two-stage process of irradiation followed by heat treatment. Colourless or near-colourless topaz rough is irradiated — using gamma rays, electron beams, or neutron bombardment — to induce colour centres within the crystal lattice, then annealed at controlled temperatures to stabilise and refine the resulting hue. This process is well-established, fully disclosed within the trade, and considered permanent under normal wearing conditions. The treatment produces three broadly recognised commercial grades of blue:

• Sky Blue: A light, airy blue comparable to a clear sky, typically produced by gamma irradiation alone.
• Swiss Blue: A vivid, medium-toned blue with strong saturation, the most widely sold variety in anniversary and fashion jewellery.
• London Blue: A deep, steely blue with a slightly greenish or greyish secondary hue, produced by neutron irradiation and considered the most sophisticated of the three grades by many buyers.

Gemmological laboratories do not routinely certify treated blue topaz, as the treatment is universally assumed and the material is not traded at price points that warrant laboratory reports. Disclosure at point of sale remains the industry standard.

Physical and Optical Properties

Topaz has a hardness of 8 on the Mohs scale, making it durable for everyday jewellery wear, though its perfect basal cleavage — a single direction of easy splitting — means that stones set in rings should be protected from sharp blows. The refractive index ranges from approximately 1.609 to 1.643, and the specific gravity is typically 3.49 to 3.57. Blue topaz displays a vitreous lustre and, when well cut, exhibits considerable brilliance. The orthorhombic symmetry produces weak to moderate pleochroism, with blue topaz showing slightly differing tones of blue and colourless when viewed along different crystallographic axes.

In the Trade

Blue topaz ranks among the most commercially accessible of all coloured gemstones. Brazil remains the dominant source of topaz rough destined for treatment, with significant production also from Sri Lanka, Nigeria, and Russia. The abundance of material and the efficiency of the treatment process keep retail prices modest relative to other blue gemstones such as sapphire or aquamarine, making it an attractive option for anniversary jewellery across a wide range of budgets. It is commonly faceted in standard commercial cuts — oval, cushion, pear, and round brilliant — and is frequently set in sterling silver or white gold to complement its cool blue tones.

Traditional Alternatives

Older anniversary lists, including those of British and American origin predating the mid-twentieth century, associate the fourth anniversary with linen, silk, or fruit and flowers rather than any gemstone. These traditional designations persist in some cultural contexts, but the modern gem list — with blue topaz as the fourth-anniversary stone — has become the dominant reference in contemporary jewellery retail and gifting.