(IDEX Online) - Christie's says a 24.45-carat heart-shaped diamond is the highest-valued jewel it has ever offered at an online auction.The D color, IF, Type IIa diamond pendant (pictured) has an estimate of of $1.55m to $2.55m.The gem leads the Jewels Online: Summer Sparkle sale, which closes on 18 August.Christie's previous record estimate for a high-value jewel online was $1m to $2m for a 28.86-carat D-color, emerald-cut, VVS1 Type IIa diamond. It sold for $2,115,000 after 31 bids, setting a new record for any jewel sold in an online auction.Another highlight of the ongoing online sale is The Summer Sunrise, a fancy vivid orange yellow diamond ring of 9.83 carats, VS1 clarity (estimate $600,000-800,000).