White topaz and white sapphire are two very similar precious stones. However, the two of them have several subtle differences in their composition as well as their appearances that might escape the naked eye. It is these differences that make the two stones so different from one another.
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Topaz is a condensed silicate mineral that is composed of aluminum and fluoride. The stones mostly grow in crystal form and end up being in prismatic shapes. Topaz in its purest form is completely clear. In this state, the topaz is called "white topaz," even though the topaz is devoid of pigmentation. White topaz is the rarest form of topaz because the crystal is very prone to impurities during creation which results in pigmentation. Topaz is an eight on Mohs' scale of mineral hardness, with 10 being the hardest, making topaz one of the hardest minerals in existence.
Sapphire usually comes in a blue color, but it can be white in color if it has more oxygen in it than a normal sapphire. Sapphire is also very similar to rubies in composition with the only difference being the pigmentation. Sapphires are composed of aluminum oxide as well as iron, titanium and chromium. Sapphires are very rare in the white color because the oxygen allowed in the crystal is kept very low. When sapphires are white, they do contain pigmentation of a yellowish hue, but are still called "white."
Because white topaz is much more common than white sapphire, topaz is found in jewelry much more frequently than sapphire. Sapphire is used in jewelry, just not in the same volume. More often than not, sapphires will be used as center stones for ornate necklaces or bracelets, while white topaz would be used as secondary ornamental stones. Topaz would only be used as a center piece if the other stones were of a more common quality that white topaz.
White topaz ranges from about $50 to $100 per carat, whereas white sapphire is closer to $200 to $300 per carat. This is because the rarity of white sapphire is much higher than that of white topaz. White topaz is also slightly more susceptible to scratching or breaking than white sapphire. The potential for longevity of a stone greatly affects the price as well.