* Rarity: Historically, the perceived rarity of a gemstone significantly influenced its value. Some emeralds or rubies of exceptional quality and size can be rarer than diamonds of comparable size and quality.
* Quality: The quality of each gem, including its color, clarity, cut, and carat weight, plays a huge role in determining its value. A flawless, vividly colored ruby or emerald can be worth more than a lower-quality diamond.
* Market Demand: Market demand fluctuates based on trends, fashion, and cultural preferences. Currently, diamonds generally have a wider and more consistent market demand for engagement rings and other jewelry.
* Size and Carat Weight: Larger stones of any gem variety are generally more valuable than smaller ones, assuming comparable quality.
General Trends
* Diamonds: Diamonds benefit from sophisticated marketing and grading systems, making it easier to establish value. Large, high-quality diamonds can fetch extremely high prices.
* Rubies: Fine-quality rubies with a vibrant "pigeon blood" red color are highly sought after and can command prices per carat that rival or exceed those of diamonds.
* Emeralds: Emeralds are known for their vibrant green color, but they are often included (have internal imperfections). Large, flawless emeralds with intense color are extremely rare and valuable.
Conclusion
There is no definitive answer as to which of the three is the most precious. A top-quality ruby or emerald could potentially be worth more than a lower-quality diamond. It all comes down to the specific quality, rarity, and market demand for each individual stone.
If you were to press me for a very general, broad answer, considering market trends and the availability of high-quality examples, diamonds generally have a wider and more consistent market demand and perceived value, making them arguably the most commonly considered "precious." However, *exceptional* rubies and emeralds can be more valuable than *average* diamonds.