Match 11 of #MinCup23 today and this will be a tough one: shiny mineraloid #opal against good, old bubbling #calcite. Opal has the looks but I prefer a crystal structure... See some fabulous examples of #calcite to convince you...

#Calcite can form very well-defined crystals but sometimes also looks a bit plain and beige. All samples from mineral exhibition at Uni Münster.