(IDEX Online) - Christie's is auctioning its most valuable ever lot online- a spectacular type IIa 28.86-carat diamond with a guide price of up to $2m.The incredibly rare, near-flawless, emerald-cut stone is VVS1 clarity, and the largest D-color stone Christie's has ever offered in an online-only sale.The auctioneers say increased online interest has encouraged them to offer more valuable items in their monthly sales."Year-over-year, we have seen an increase in online participation and the value threshold for transacting online," said Rahul Kadakia, international head of jewelry."Recognizing greater client confidence, we are proud to announce the highest valued lot ever to be offered for sale in our June 2020 Jewels Online auction." Only two per cent of diamonds are type IIa, boasting an unrivaled color and transparency due to their chemical purity.Christie's Jewels Online auction takes place from June 16 to 30. The diamond's estimate is $1m to $2m.