Gemstone value of the stone. Some treatments are used widely because the resulting gem is stable, while others are not accepted most commonly because the gem color is unstable and may revert to the original tone. Heat can improve gemstone color or clarity. The heating process has been well known to gem miners and cutters for centuries, and in many stone types heating is a common practice. Most citrine is made by heating amethyst, and partial heating with a strong gradient results in “ametrine” – a stone partly amethyst and partly citrine. Aquamarine is often heated to remove yellow tones, or
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