LONDON, England, UK (Reuters) — It’s said to be the largest blue topaz gemstone ever mined and glitters a vivid electric blue in the light. Now it will be shown off to the world after three decades kept hidden away in a vault. The Ostro stone is an oval 9,381 carat blue topaz, equivalent in weight to two kilos or two bags of sugar. It’s being given on permanent loan to a London museum by the […]