(IDEX Online) - The USA's only active diamond mine is celebrating its 50th anniversary with limited edition replicas of its most valuable find - a $1m-plus gem.The Crater of Diamonds State Park, near Murfreesboro, in Arkansas, charges day trippers $10 a day to sift the soil - and lets them keep any gems they find.In 2015, Bobbie Oskarson, from Colorado, found a rough, icicle-shaped 8.52-cts stone, after just 20 minutes.It was later cut into a highly unusual 4.60-cts D / IF triolette-shaped gem, with 147-facets named Esperanza (Spanish for "hope"). The diamond now belongs to investors and is reckoned to be worth over $1m.Thirty-five cubic zirconia replicas, in a silver setting, are being sold by the park at $500 each.