Corundum Color Standards
Corundum is a very prominent and popular gemstone on the market. Corundum can come in numerous hues, such as a red variation named "Ruby", a blue variation called "Blue Sapphire", a yellow variety called "Yellow Sapphire", a pinkish orange, to an orangy pink type called "Padparadscha". The price of each stone is determined by its quality, which is reflected by the 4Cs: Color, Clarity, Cut, and Carat. Color is regarded as the most essential aspect in the pricing structure of the Corundum. As a result, the grading system must be very carefully controlled in order to achieve consistency in the results. Master set, standard light source, and grading ambient are key aspects.
The Gem and Jewelry Institute of Thailand (GIT) has been conducting extensive study on "Colored Stone Quality Standards," especially on gem corundum, for over two decades. Consequently, numerous corundum master sets, such as Pigeon's Blood, Royal Blue, Cornflower Blue, and Padparadscha, have already been utilized for color grading in our trade services. To better serve the global ruby market, a new trade color known as "Rabbit's Eye Red" for ruby has been developed and implemented to fill a gap in the fine color of ruby and to commemorate "Chanthaburi" township, the world capital of the ruby trade, which has a rabbit as its provincial symbolic icon. (Remark: Chanthaburi means the city of Moon as Chandra or Chantha or Chan is meant “Moon” and Buri is the “City” in Thai – the legend of the moon was known and believed that there is a rabbit on the moon as we imagine when look up to the moon at night)
Pigeon’s Blood Red Ruby
Ruby, which is widely recognized as the most beautiful, the best quality, and the highest-priced, is a vivid red color called “Pigeon’s Blood Red”. This color is considered rare and sought-after in the market.
To be classified as Pigeon's Blood Red, a ruby must meet certain criteria, including being of natural origin. Moreover, only conventional heat treatment is permitted, and the stone must be a faceted stone weighing at least 0.20 ct; in general, Pigeon's Blood Red refers to ruby that has a vivid red (deep red hue) and exhibits homogenous color throughout the stone.
Samples of Pigeon’s Blood Red Rubies
Royal Blue Sapphire
Blue sapphires in bright to deep blue, known as "Royal Blue," are the most desired and costliest on the market. The blue sapphire entitled to designate Royal Blue hue must satisfy identical critical criterion as ruby. The Royal Blue hue refers to sapphire with a bright to strong blue color, and the stone should not have identifiable modified colors such as purple or greenish when viewed from the front (face-up) view.
Samples of Royal Blue Sapphires
Cornflower Blue Sapphire
Another highly coveted and fine quality blue sapphire, originated from Kashmir in the North of India, is in vivid blue comparable to the color of butterfly pea, partially called "Cornflower Blue". The Cornflower Blue hue refers to a vivid blue sapphire (with a slight tint of violetish modifier color allowed). Most significantly, the stone must have a velvety look as a result of light scattering from the cloudy inclusions.
A blue sapphire that qualifies to be called "Cornflower Blue," like "Royal Blue," must meet the fundamental requirements. Nevertheless, sapphires in Cornflower Blue and Royal Blue can also be found in other sources.
Samples of Cornflower Blue Sapphires
Rabbit’s Eye Ruby
Ruby with a pinkish red hue, in particular, has been a favorite color in the trade. Yet, no trade name has been assigned to this hue in order to attract the attention of customers. This color category is distinct from "Pigeon's Blood," the classic color trade phrase that has been used for generations. As a result, the GIT devised the "Rabbit's Eye" red hue to accommodate for such delicate ruby color that falls outside the Pigeon's Blood boundary. This new trade name has also been established to commemorate the Chanthaburi municipality, which is known as the "City of Gems" and is widely recognized as the world's ruby centre. Chanthaburi is also known as the "City of Rabbits" since rabbits are their provincial symbol.
Sample of Rabbit’s Eye Red Rubies
Padparadscha sapphire is a highly sought-after gemstone among gem collectors. Padparadscha sapphire color, in general, refers to sapphire with a color spectrum of orange-pink to pink-orange, which is regarded a rare hue of natural sapphire. Padparadscha's origin is taken from the Sinhala word "Padmaraga," which signifies lotus color.
According to GIT study, a sapphire that fits the criteria for the name "Padparadscha" must have a color that falls between the designated ranges of Padparadscha hues. Also, the stone must be of natural hue or be treated solely with ordinary heat (without adding beryllium). Additionally, when viewed from the front (face-up) view, the hue should be consistent. Also, color stability (as measured by a fade test) has been considered for Padparadscha qualifying.
Samples of Padparadscha Sapphires
Gem testing laboratory
The Gem and Jewelry Institute of Thailand (Public Organization)