Sapphire - Sapphire is the most precious and valuable blue gemstone. It is a very desirable gemstone due to its excellent color, hardness, durability and luster. The 423-carat (84.6g) Logan sapphire in the national museum of natural  history. in Washington D.C. is one of the largest faceted gem-quality blue sapphire in existence.

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Sapphire & Ruby
A star sapphire is a type of sapphire that exhibits a star-like phenomenon known as asterism. The Star of Adam is the largest blue star sapphire which weight 1404.49 carats. The gem was mined in the city of Ratnapura Sri Lanka. The 182-carat star of Bombay (mined in Sri Lanka) located in the national museum of natural history in Washington D.C. is another example of a large blue star sapphire. The value of a star depends not only on the weight of the stone but also the body color, visibility, and intensity of the asterism.