Topaz is a hard, transparent mineral. It is a compound of aluminum, silica, and fluorine. Gem topaz is valuable. Jewelers call this variety of the stone "precious topaz". The best-known precious topaz gems range in color from rich yellow to light brown or pinkish red. Topaz is one of the hardest gem minerals. In the mineral table of hardness, it has a rating of 8, which means that a knife cannot cut it, and that topaz will scratch quartz.   

    The golden variety of precious topaz is quite uncommon. Most of the world's topaz is white or blue. The white and blue crystals of topaz are large, often weighing thousands of carats. For this reason, the value of topaz does not depend so much on its size as it does with diamonds and many other precious stones, where the value increases about four times with each doubling of weight. The value of a topaz is largely determined by its quality. But color is also important: blue topaz, for instance, is often irradiated to deepen and improve its color. 

    Blue topaz is often sold as aquamarine and a variety of brown quartz is widely sold as topaz. The quartz is much less brilliant and more plentiful than true topaz. Most of it is a variety of amethyst: that heat has turned brown.

    黃水晶是一種堅硬、透明的礦物質。 它是鋁、硅和氟的化合物。 黃水晶寶石價值不菲。 珠寶商把這種石頭稱為"黃玉"。 最出名的黃玉有各種顏色如深黃色、淡棕色、淺紅色等。 黃水晶是最堅硬的寶石礦中的一種。 在礦石硬度表上，它的硬度為8，這表明刀子不能割開它而它可在石英上劃痕。 金黃色的黃玉品種非常罕見。 世界上大多數的黃水晶是白色或藍色的。 這些白色或藍色的黃水晶晶體很大，常常有數千克拉重。 由于這個原因，黃水晶的價值不像鉆石和許多其它寶石那樣主要依賴于其大小，重量翻一番價值即上升約四倍。 黃水晶的價值很大程度上取決于其品質，但顏色也很重要。 舉例來說，藍色的黃水晶常需放射處理以加深和改善其顏色。 藍色的黃水晶常被作為海藍寶石出售，許多種棕色石英被當作黃水晶廣為販賣。 石英光亮度遠小于黃水晶，礦藏儲量也遠較黃水晶豐富。 大多數石英是一種紫水晶，高溫使其變為棕色。