Gemstone usually found in. For example, diamonds, which have a cubic crystal system, are often found as octahedrons. Gemstones are classified into different "groups", "species", and "varieties". For example, ruby is the red variety of the species corundum, while any other color of corundum is considered sapphire. Other examples are the emerald (green), aquamarine (blue), red beryl (red), goshenite (colorless), heliodor (yellow) and morganite (pink), which are all varieties of the mineral species beryl. Gems are characterized in terms of refractive index, dispersion, specific gravity, hardness, cleavage, fracture and luster. They may exhibit pleochroism or double refraction. They may have luminescence
Varieties of Arabic sedentary varieties and the rural varieties within the Arabian peninsula are even more conservative than those elsewhere. Within the sedentary varieties, the western varieties (particularly, Moroccan Arabic) are less conservative than the eastern varieties. A number of cities in the Arabic world speak a "Bedouin" variety, which acquires prestige in that context. When it comes to phonetics the Arabic dialects differ in the pronunciation of the short vowels (, and ) and a number of selected consonants, mainly , and the interdental consonants , and , in addition to the dental . Emphasis spreading is a phenomenon where is backed