A very rare flawless pink diamond to headline Sotheby’s autumn sale in Hong Kong will become one of the most valuable gems ever sold if it reaches its low-bid estimate of $1.5m per carat.

While white diamond prices have steadied in recent years, rising interest in rarer coloured diamonds has pushed prices towards record levels.

Sotheby’s expects its 8.41-carat pear-shaped internally flawless fancy vivid purple-pink diamond to fetch between $12.8m and $15.4m at auction on October 7. If the rare diamond sells at the top of that range, it would put it among the top three pink stones ever sold at auction on a per-carat basis.