Silvestri's ability to spot the "life" - or rather, the unique characteristics - within stones comes from years of expert training. Unlike the "four Cs" - colour, clarity, cut and carat - system for rating diamonds, assessing coloured gemstones is a more complicated and often controversial process.

Despite synthetic and artificially enhanced stones posing challenges to the trade, market confidence in coloured gemstones has grown significantly in the past few years thanks to transparent valuation standards, the establishment of institutional gemology labs and active market transactions.

"Looking back at our auctions in the past few years, I would say [natural] coloured gemstones have dominated," says Vickie Sek, director of jewellery for Christie's Asia.

"Buyers are really going for the beautiful natural gemstones. Most of the top 10 lots are emeralds, rubies and sapphires."

Graeme Thompson, director of jewellery for Bonhams Asia, agrees. "We predicted 2015 to be the year of coloured gemstones, and results from the sales definitely backed that. There's no doubt that the supply of fine gemstones is diminishing, but the demand is increasing. The more the market develops, the more people understand gemstones and have the confidence to buy."

Sapphires and rubies have outpaced diamonds from 2006 to 2013. According to the Gemval valuation guide, prices for rubies rose by 47 per cent, while the RapNet Diamond Index indicates that prices for 1ct diamonds have increased at a lower rate - 32 per cent since 2006. And luxury maisons such as Bulgari, Cartier, Van Cleef & Arpels and Chanel have been incorporating more coloured gemstones into their jewellery pieces, from rubies and emeralds to sapphires and spinels.

"Colour is the most important [for Bulgari]," Silvestri says. "It has to be brilliant and stylish."

Chanel's latest talisman-themed high-jewellery collection features sugarloaf-cut and round-cut blue tanzanites, orange topazes, yellow sapphires and red spinels.

"Gabrielle [Coco] Chanel owned many coloured stones in her personal jewellery collection," says Chanel's international fine jewellery director, Benjamin Comar. "She loved to create jewellery for herself and her friends."

Aesthetics aside, rarity has been driving the growing demand for coloured gemstones.

"If you follow the wholesale market for years, there's always a certain amount of diamonds; however, there are varying amounts of coloured stones," says jeweller Edmond Chin. He designed an exquisite necklace featuring Burmese rubies that was auctioned by Christie's Hong Kong in June for over HK$100 million, setting a world-record auction price for a ruby necklace.

"Important [gemstone] deposits are becoming mined out," Chin adds. "All mines will come to an end. Naturally good-looking gemstones are ultra-rare."

According to Pia Tonna, business relations director for the London-based miner Gemfields, top-quality stones are hard to uncover. "We mine 30 million carats a year, and only 7 per cent are top-quality," she says. "[Coloured gemstones] are still super-rare despite the emergence of new deposits. Once the mines are finished, they're finished - they'll never produce any more."

Collectors and connoisseurs have been quick to pick up on the trend. "Our male collectors are not just buying coloured gemstones for their wives to wear but also to collect for their own investments," Sek says.

Thompson has noticed that collectors who once focused on diamonds are growing more confident about coloured gemstones. "Diamonds are a good starting point as they are beautiful, and easy to wear, understand and research," he says. "But once [collectors have] made a few diamond purchases, they want to build up variety in their collection."

Education and awareness are contributing to the growing appetite for coloured gemstones.

"Clients realise that these beautiful stones are getting rarer," says Pierre Rainero, Cartier's head of heritage. Geographical origins such as Burma [Myanmar] for rubies and Kashmir for sapphires, which people associate with strong provenance, make the stones even more sought-after."

While the diamond industry enjoys centralised control by conglomerates such as De Beers, the sourcing of coloured gemstones is often volatile. The places known for having the best-quality stones - Myanmar, Kashmir, Colombia and Madagascar - tend to have political and economical turbulence. Gemfields - which owns the world's largest emerald mine, Kagem in Zambia, along with the Montepuez ruby mine in Mozambique - attempted to shake up the industry in 2009 by launching a "mine-to-market" campaign to create more transparency and consistency.

Chin believes greater transparency in the coloured gemstones trade has helped to restore market confidence. "Right now there's more transparency and knowledge," he says. "For example, up to three years ago, there were rarely talks about the types of enhancements in emeralds, be it synthetic resins or traditional [cedarwood] oil."

Auction houses have also adopted full disclosure policies, informing customers whether gemstones have been artificially enhanced.

According to Sek, important stones put on the auction block require certifications from at least three different independent gemology laboratories such as the Gübelin Gemmological Laboratory, GemResearch Swisslab and the Gemological Institute of America.

While the judgment of colours can be highly subjective, laboratories worldwide are attempting to introduce a uniform grading system. For example, pigeon's blood for rubies and cornflower blue for sapphires are considered top quality and highly desirable gemstone hues.

Gemfields has also introduced a grading system for rough emeralds.

Encouraged by the growing interest in coloured gemstones, luxury jewellers have taken more creative licence in adding colours to their collections.

"More high jewellers have woken up to colours because they realise there's much more competence in the market. It gives designers a much bigger scope to be more creative," Tonna says.

Comar adds: "The challenge [now] is to use coloured stones wisely, with a legitimate design project, in order to preserve the rarity and quality expected from a high jewellery piece. I strongly believe that the design should be nothing but the true expression of the identity of a brand."

"At Bulgari, we always start from the coloured gems," Silvestri says. "One of the first things that I learned from this is to buy gems only if you can imagine something that you can do with the gemstones."

Industry veterans are optimistic about the continuing popularity of coloured gemstones due to the projected growth of burgeoning markets such as China, India and Southeast Asia. "Wealth is increasing in Asia," Thompson says.

Meanwhile, new deposits are being discovered in Mozambique ruby mines and Zambian emerald mines. However, experts don't believe that increased production will affect the investment value of coloured gemstones.

"Don't forget, the [sapphire] mines at Kashmir were once new," Chin says. "It's really down to the quality of stones. Sometimes it's a good thing when the market opens up. Without transactions of gemstones, it will be difficult to mark the price, and people won't have the confidence [to buy]."