DUBAI (REUTERS).- Auction house Christie’s will put one of the world’s largest pearls on the block at its Dubai spring jewelry sale on April 20 in a nostalgic reminder of the Emirate’s pearl diving past. The auction house is showcasing an array of pearls, diamonds, gems and gold jewelry ahead of the sale. Pearls are especially unique to Dubai, as pearl hunting was a major source of income a century ago in the port city. Christie’s — who have been holding regular auctions in the Middle East since 2006 — saw regional sales reach $51 million last year, up from recession lows of $18-20 million in 2009. Globally, the art and jewelry-selling powerhouse posted record sales of $5 billion in 2010. “2009 was a very difficult year for us. Volumes were down and we had price softening,”