硬水鋁石（Diaspore），英文名來自希臘文，意指「分散」，因其加熱後會爆裂四散。這種水合鋁氧化物礦物屬斜方晶系，與針鐵礦為異質同構礦物，晶體呈扁平狀，聚晶呈片狀或鱗狀晶簇，解理發達，破裂面常有珍珠光澤。硬水鋁石通常為無色、灰色、淡黃色，稀有紫色也可見。觀賞價值高的變色硬水鋁石，產自土耳其，商業上稱為「舒坦石」（Zultanite），其變色效應不亞於亞歷山大石，變色效果包括綠色、粉紅色、黃色和紫色，變色效應強的大克拉數標本極為稀有，難以估價。
在工業上，硬水鋁石廣泛存在於鋁土礦、紅土和某些岩石中，可用作耐火材料和鋁的提煉。硬水鋁石硬度高，不易刮擦，但因解理發達，需避免碰撞和接觸刺激性化學物質，如家用清潔劑和香水，這些可能會損害寶石。
Charoite, a rare purple silicate mineral, is characterized by its radiating fibrous metamorphic structure, which resembles a group of entwined dragons, hence earning it the commercial name Purple Dragon Crystal. The other name, Charoite, is derived from its discovery site along the Chara River in Russia. In terms of color, the only gemstone similar to it is Sugilite, but the two can be differentiated by their crystal structures. A gemstone with a similar fibrous structure is Seraphinite, though its composition is entirely different from that of Charoite.
Calcite, also known as “Iceland spar,” exhibits a wide range of colors and appearances, often being used as an imitation for other gemstones. For collectors of fluorescent stones, calcite is an excellent choice; its strong birefringence plays a crucial role in the optical components of gemstone identification instruments.
Cleavage refers to the property where a crystallized mineral breaks along weaker planes within the crystal structure under external force, forming smooth surfaces. In the gemstone industry, the cleavage characteristic is utilized by striking along the cleavage plane before cutting and polishing. Cleavage is classified into five grades based on the degree of perfection: perfect cleavage, good cleavage, fair cleavage, poor cleavage, and no cleavage. Cleavage is not only a critical factor in gemstone design but also serves as an important indicator in gemstone identification. Cleavage is an inherent property of gemstones, and even perfectly crystallized diamonds have cleavage planes.
Apatite, whose name is derived from the Greek word meaning “deception,” is so named because its color and luster often resemble other gemstones. Apatite is a group of calcium phosphate minerals that contain chlorine or fluorine, with the main types being fluorapatite and chlorapatite. It comes in a variety of colors, including colorless, yellow, blue, purple, green, brown, and white, and may exhibit phosphorescence and strong pleochroism. Apatite is found in igneous, metamorphic, and sedimentary rocks, with gem-quality apatite mainly coming from pegmatites and alluvial deposits. Despite its relatively low hardness, making it less favored by jewelers, apatite is an important industrial mineral for extracting phosphorus, which is widely used in the medical, chemical, and other industries. When selecting apatite gemstones, flawless and brightly colored stones are the most desirable, especially those with a cat’s eye effect. While apatite is resistant to impact, it is susceptible to damage from acids, so proper care must be taken to avoid exposure to acidic substances.
Danburite is a kind of rare gem, the English scientific name comes from its discover location—Danbury, Connecticut, USA, the Chinese name is derived from its appearance, which is similar to topaz. It’s primarily found in pegmatite and metamorphosed carbonate rocks that are rich in cryogenic hydrothermal fluids, coexist with plagioclase and orthoclase in dolostone. Danburite can be found in colors ranging from colorless, light yellow, brown, to the rarer pink and purple. Commercially, the appearance and index of refractive of danburite are both similar to those of topaz, but danburite can be identified by its low specific gravity, fluorescence and low birefringence index. Most danburite appear gray and opaque, with gem-quality transparency being extremely rare, but even so most enthusiasts still value its natural appearance and regard it as collection. Danburite has high hardness and toughness, doesn’t require special maintenance in both collection and wear, makes it relatively easy to care.
Aragonite, named after Aragon, Spain, is composed mainly of calcium carbonate with manganese and iron. Despite its similarity to calcite, it is rarer in nature and less chemically stable, often converting into calcite. Both onyx and ammolite are aragonite-based gemstones, with onyx being used for decorative purposes, and ammolite admired for its vibrant colors and pattern distribution.
The English name “Kyanite” for blue kyanite comes from the Greek word meaning “blue.” Kyanite is a regional metamorphic mineral belonging to the triclinic crystal system, and its colors are often vibrant blue but can also include light blue, dark blue, and other shades. Due to its varying hardness along different axes, it is known as the “two-hardness stone.” Although kyanite is not highly valued commercially, its color rivals that of premium sapphires. Ideal selections have vivid colors and appear flawless to the naked eye. In industrial applications, kyanite is highly refractory and used to produce advanced refractory materials and ceramics, widely utilized in industries such as automotive and aerospace.
Andalusite is a metamorphic mineral with the chemical composition of aluminum silicate (Al₂SiO₅) [Al2SiO5] .Under varying temperature and pressure conditions, it can transform into kyanite or sillimanite, making these minerals polymorphs. Andalusite becomes mullite, a high-alumina material, at 1380°C and can withstand temperatures as high as 1800°C. It exhibits strong pleochroism, showing red, yellow, and green when viewed from different angles. When selecting andalusite, those with bright colors and pronounced pleochroism are preferred. Large stones over 2 carats are rare and considered collectible. Its Mohs hardness is 7.5, and it is typically reddish-brown and highly transparent. Andalusite crystals showing a “black cross” pattern on the surface are known as chiastolite.
Tanzanite was discovered in 1967 in northern Tanzania and was initially mistaken for sapphire before being recognized as a new gem. Gem-quality blue tanzanite was promoted by Tiffany & Co., with deep blue and indigo hues being the most prized. Tanzanite exhibits strong pleochroism, meaning its color can vary depending on the cutting direction. In Taiwan, tanzanite entered the market around 1995, initially as a substitute for sapphire due to its similar color. Thanks to Tiffany’s efforts, tanzanite has since gained recognition as a unique and rare blue gemstone, with prices around NT$10,000 to NT$15,000 per carat.
