Gemstone the stone such as color zoning (the uneven distribution of coloring within a gem) and asteria (star effects). The Greeks, for example, greatly valued asteria gemstones, which were regarded as powerful love charms, and Helen of Troy was known to have worn star-corundum. Aside from the diamond, the ruby, sapphire, emerald, pearl (not, strictly speaking, a gemstone), and opal have also been considered to be precious. Up to the discoveries of bulk amethyst in Brazil in the 19th century, amethyst was considered a "precious stone" as well, going back to ancient Greece. Even in the last century certain stones such
Oltu stone Oltu stone Oltu stone () is a kind of jet found in the region around Oltu town within Erzurum Province, eastern Turkey. The organic substance is used as semi-precious gemstone in manufacturing jewellery. Oltu stone, sometimes called also "Erzurum stone", is principally mined in the villages northeast of Oltu town, around Tutlu Dağı (Yasak Dağ) as well as in Alatarla, Hankaskışla and Çataksu villages. The mountainous, rough region with steep slopes is high. Oltu stone beds are formed when fossilized trees are subject to diastrophism resulting in folding. Beds of this organic substance are in thickness. Extraction is done by