Semi-precious Stones
In gemology, gemstones are divided into precious stones and semi-precious stones. Apart from precious stones such as diamonds, rubies, sapphires, and emeralds, all others are classified as semi-precious stones.
Crystal
Crystal, belonging to quartz, has a chemical composition of silicon dioxide. Crystal has a Mohs hardness of 7 with a vitreous luster.
Many crystals contain various trace metal elements, resulting in different colored crystals such as milky white called milk quartz, red called rose quartz, purple called amethyst, yellowish-brown called smoky quartz, and black called morion.
Agate
Agate, belonging to cryptocrystalline quartz, has a chemical composition of silicon dioxide. Agate has a Mohs hardness of 7 with a vitreous luster.
Agate often contains trace metal elements such as iron, manganese, and nickel. Due to pigment ions and impurities, agate displays a rich variety of colors including red, blue, green, brown, purple, gray, and black. Cross-sections reveal concentric circles, wave patterns, layers, or parallel bands in various colors such as gray, white, red, green, light brown, and light blue. There is a saying: 'A thousand types of agate, ten thousand types of jade.'
Tourmaline
Tourmaline, also known as electrical stone, is a borosilicate mineral. Tourmaline has a Mohs hardness between 7 and 7.5 with a vitreous luster.
Tourmaline is crystal clear, and the more transparent it is, the better the quality. More precious and valuable are bi-color or multi-color tourmalines that display two or more colors on the same stone. Watermelon tourmaline with red interior and green exterior is also quite precious.
