Gubelin Gemmology

Have you ever wondered why gemstones are weighed in carats? And why one carat equals exactly 0.20 grams?

The modern metric carat was standardized in 1907 at exactly 200 milligrams, or 0.20 grams. But the origins of the word carat go much further back.

The term is derived from the carob tree (Ceratonia siliqua). In ancient times, carob seeds were used as counterweights on balance scales when weighing precious materials such as gemstones. Their relatively consistent size made them useful for this purpose, although historical carat weights varied from region to region.

Today, one carat is precisely defined as 0.20 grams, and the carat is the standard unit of weight for gemstones.

So, the next time you hear the word carat, remember that its origins can be traced back to something as simple as a tiny carob seed.

Join our Gübelin Academy courses in October and November 2026 and deepen your knowledge with expert instruction.

Upcoming Courses | October/November 2026

29–30 October | Level 1 (English)
2–6 November | Level 2 (English)
9–13 November | Level 3 (English)
16–17 November | Practice Days (English)

To register, contact us at: [email protected]

Photo: Carob Seeds, Gübelin Gem Museum

What turns a pale, milky sapphire into a striking blue gemstone? We take a closer look at Geuda sapphire and the transformative effect of heat treatment.

Geuda is a type of corundum, typically found in Sri Lanka’s gem-bearing gravels. It is usually semi-transparent to translucent, with whitish, brownish, milky, or silky appearances. Heat treatment can transform this material into beautiful and marketable gems. For example, Sri Lankan Geuda sapphires can develop striking blue colours when heated, significantly enhancing their beauty and appeal.

While heat treatment can produce beautiful sapphires, a sapphire with a comparable natural colour would generally be more valuable than one whose colour has been enhanced through treatment. This is why treatment disclosure is essential, allowing buyers to understand the gemstone’s characteristics and make informed decisions. Today, the majority of sapphires are heat treated.

Our Level 2 course explores these topics in greater depth, helping you build the knowledge and skills to confidently understand gemstones.
To book your place, get in touch with us at: [email protected]

Next Courses:

Lucerne, Switzerland (English)
10-11 September Level 1
14-18 September Level 2
21-25 September Level 3
28-29 September Practice Days
29-30 October Level 1
2-6 November Level 2
9-13 November Level 3
16-17 November Practice Days

Hong Kong (Mandarin)
10-11 September Level 1
21-25 September Level 2 - fully booked
29-30 September Level 1
5-9 October Level 3
21-22 October Level 1
26-30 October Level 2

Photo: Rough Sapphire from Sri Lanka, Gübelin Gem Museum Wall

Cornflower Blue - a new trade colour for Gübelin

We are pleased to announce the introduction of Cornflower Blue beginning January 1st 2027, a trade colour available for Gübelin reports for natural blue sapphires of exceptional colour and visual quality

Inspired by the characteristic blue of the cornflower (Centaurea cyanus), the term cornflower blue has long been used within the gemstone trade, although interpretations of its exact definition vary.

With Cornflower Blue, the Gübelin Gem Lab establishes a stringent and clearly defined set of criteria for this coveted trade colour.

To qualify for Cornflower Blue for Gübelin, a sapphire must meet the following criteria:
• Natural blue sapphire from any geographical origin
• No signs of treatment, including heat or clarity enhancement
• Saturated blue colour with a light to moderate tone, possibly with a gentle hint of violet
• Homogeneous colour distribution
• High level of transparency; a slight velvety appearance caused by subtle light scattering is welcome, though not compulsory
• No eye-visible inclusions and no dark inclusions
• No major windowing or extinction
• Lively colour reflections (when faceted)

The designation Cornflower Blue for Gübelin is independent of geographical origin and is assessed based on the individual visual characteristics and quality of the sapphire. Only a small proportion of blue sapphires are expected to fulfil all required criteria.

From January 2027, qualifying sapphires will be eligible to receive the Cornflower Blue on reports issued by the Gübelin Gem Lab.

Clients who have submitted a sapphire to the Gübelin Gem Lab within the past five years may request a reconsideration of their stone for the Cornflower Blue. For eligible stones, the reconsideration service will be offered at a 75% discount on the applicable service fee.

The Gübelin Gem Lab will offer express on-site testing service for all major gemstones with no express charge in Jewellery & Gem World Hong Kong 2026 .

We are looking forward to meeting you at 3H00, Asia World Expo Hong Kong (AWE). Our opening hours:

14th - 18th Sep, 2026: 10:00 a.m. - 6:00 p.m.
18th Sep, 2026: 10:00 a.m. - 5:00 p.m.

For Hong Kong Convention and Exhibition Centre (HKCEC), our location is 3M209

Our opening hours: 🕔
16th – 20th Sep, 2026: 10:00 a.m. – 6:00 p.m.
20th Sep 2026 : 10:00 a.m. – 5:00 p.m.

You are welcome to submit your gemstones to enjoy our express services. Goods submitted by 2pm at our booth or 5pm at our office in Central, may be collected in late afternoon after two days.

During the fair period, we equally accept goods directly at our Gem Lab in 1004-1005, 10/F., China Building, Central, Hong Kong from Sep 14th until Sep 21st , 2026, office hours in Central are from 9:00am to 6:00pm.
Please book an appointment in advance by email: [email protected] or call:
+ 852- 2868 2781.

We look forward to greeting you in Hong Kong.

The shape and carat weight of a cut gemstone are usually influenced by the shape of the rough. The crystal habits of rough sapphire can be bipyramidal, prismatic, or tabular. During the cutting process, the cutter needs to select the most appropriate cut according to the characteristics of the rough in order to maximize the gemstone’s weight while achieving the best possible colour.

Furthermore, the proportions and symmetry of the cut are also very important. If the pavilion is cut too shallow, the gemstone may appear colourless in the centre, a phenomenon known as a “window.” Conversely, if the pavilion is cut too deep, the gemstone may appear dark or dull around the edges, a phenomenon known as “extinction.” Therefore, sapphire cutting requires a high level of experience and skill on the part of the cutter.

Photo 1: Prismatic habit with basal terminations
Photo 2: Tabular habit

Would you like to learn more but wonder where to start?

From the comfort of your home, you can join our online “Basics” courses. These courses provide an introduction to the gemmological, practical, and historical fundamentals of ruby, emerald, and sapphire.

Explore our online ‘Basics’ courses (link in bio)