Natural Precious Gemstone Handbook: Characteristics, Hardness, Birthstones, and Buying Guide for 13 Gemstones

The four traditionally recognised precious gemstones in the international jewellery industry are diamond, ruby, sapphire and emerald. When chrysoberyl cat’s eye is included, they are collectively known as the “five precious gemstones”. They combine rarity, historical standing and globally recognised marketability and collectible value.

Mohs hardness indicates a gemstone’s resistance to scratching. Quartz dust commonly found in the air has a hardness of approximately 7, so gemstones with a hardness of 7.5 or above (diamond 10, ruby and sapphire 9, spinel 8, emerald and aquamarine 7.5 – 8) are less likely to be scratched during everyday wear and are suitable for rings. Softer gemstones, such as tanzanite 6 – 7 and opal 5 – 6.5, should be protected from impacts and friction and are better worn as pendants or earrings. Although emerald is not particularly soft, it often contains inclusions and has lower toughness, so it likewise requires protection from impacts.

The birthstones covered in this guide are: January garnet (including garnet varieties such as tsavorite and spessartine), March aquamarine, April diamond, May emerald, July ruby, August spinel, September sapphire (including Padparadscha), October opal and December tanzanite. For the remaining months, please refer to the WILLS Birthstone Guide.

Famous origins, such as Kashmir sapphires, Burmese Mogok rubies and Colombian emeralds, have unique geological conditions. Combined with their historical reputation and declining mine output, this generally gives them a market premium. However, origin opinions are not a guarantee of quality; top gemstones should still be assessed holistically based on colour, saturation, clarity, cut, treatment status and certification.

Heating (commonly called “burning”) is a widely accepted treatment in the corundum market for rubies and sapphires, used to improve colour and transparency. Unheated gemstones retain their naturally formed state and, when they have excellent colour and clarity, are usually rarer and more expensive. When buying, the most important things are clear disclosure of treatment and verification through a trusted laboratory certificate.

Emeralds have a higher historical standing and a more collectible character, but they often contain inclusions and are commonly oil-treated. Tsavorites are garnets with stronger fire, higher clarity and generally no treatment, making them easier to care for daily. Choose an emerald for a classic fine-jewellery feel; consider a tsavorite if you value transparency, fire and everyday durability.

The WILLS JEWELLERY Central private showroom is located at Room 1003, 10/F, Tak Wo Building, 17-19 D'Aguilar Street, Central (1 Wo On Lane), Hong Kong, and operates by appointment only. You can book online or WhatsApp +852 3571 9629 to specify the gemstone types, colour ranges and budget you would like to compare. A consultant will arrange side-by-side comparisons under standard lighting and provide bespoke consultation.