洮(tao)石(shi)又稱"洮(tao)河石(shi)",以產(chan)于甘肅(su)省古(gu)洮(tao)州(zhou)的洮(tao)河沿岸(an)而(er)得名(ming)。洮(tao)石(shi)呈綠藍色,近(jin)似(si)綠松石(shi)的顏色,多帶(dai)回形(xing)、曲形(xing)紋,質地(di)細膩(ni)但不堅,當時人稱其為洮(tao)硯。它制成的硯臺稱"洮(tao)石(shi)硯"或"洮(tao)硯",或"洮(tao)河石(shi)硯"等。質地(di)細膩(ni)、致密,晶瑩(ying)而(er)有(you)光澤(ze),紋路猶如翻騰的海浪。
洮(tao)(tao)石的具體產地在甘肅省甘南藏族(zu)自治州卓尼縣洮(tao)(tao)硯鄉約8公(gong)里的洮(tao)(tao)河東岸。
化(hua)(hua)(hua)學(xue)分(fen)析表明,洮石含二氧(yang)(yang)化(hua)(hua)(hua)硅(gui)60.40%,二氧(yang)(yang)化(hua)(hua)(hua)鉈0.72%,三氧(yang)(yang)化(hua)(hua)(hua)二鋁(lv)15.54%,三氧(yang)(yang)化(hua)(hua)(hua)二鐵(tie)7.74%,氧(yang)(yang)化(hua)(hua)(hua)鈣(gai)1.19%,氧(yang)(yang)化(hua)(hua)(hua)鎂4.56%,氧(yang)(yang)化(hua)(hua)(hua)鉀(jia)4.30%,氧(yang)(yang)化(hua)(hua)(hua)鈉(na)0.98%.比(bi)重約2.7-2.9,硬度約3-4.洮石屬泥盆系(xi)中(zhong)變質細(xi)泥板頁(ye)巖(yan),結構(gou)細(xi)密,且(qie)含多種金屬離子,石質瑩潤如(ru)玉,叩之無(wu)聲,呵之出水(shui)珠,用(yong)(yong)以制硯,貯水(shui)不(bu)(bu)耗,歷(li)寒不(bu)(bu)冰,澀不(bu)(bu)留筆,滑不(bu)(bu)拒墨,且(qie)硬度適中(zhong),具有發墨快(kuai),研墨細(xi),不(bu)(bu)損筆,揮撒自如(ru),濃淡相宜(yi),用(yong)(yong)之得心應手(shou),風(feng)雅盡賦等(deng)特點。
洮石(shi)(shi)(shi)除了綠(lv)(lv)洮石(shi)(shi)(shi)之外,還有(you)一種紅洮石(shi)(shi)(shi),色彩有(you)些濃艷,比較少,不如(ru)綠(lv)(lv)洮石(shi)(shi)(shi)常見(jian)。但綠(lv)(lv)色石(shi)(shi)(shi)硯(yan)并(bing)非(fei)洮硯(yan)一家(jia),綠(lv)(lv)端硯(yan)、綠(lv)(lv)松花石(shi)(shi)(shi)硯(yan)、綠(lv)(lv)賀蘭石(shi)(shi)(shi)硯(yan)、綠(lv)(lv)易縣(xian)石(shi)(shi)(shi)硯(yan)等(deng)各有(you)千(qian)秋,區別時要(yao)細審紋理,并(bing)看是否有(you)綠(lv)(lv)洮石(shi)(shi)(shi)中(zhong)常見(jian)的微量黑色墨筋或墨點。
制作洮(tao)(tao)硯的(de)洮(tao)(tao)石(shi)有數種,一是鴨頭綠(lv),也稱"綠(lv)漪石(shi)",色澤(ze)綠(lv),有水波狀紋路(lu),石(shi)質堅細,瑩(ying)潤如玉,是洮(tao)(tao)石(shi)上品。如在(zai)綠(lv)色紋路(lu)中(zhong)夾雜黃色痕跡者,則更名貴。二是鸚鵡綠(lv),色澤(ze)深綠(lv),石(shi)質細潤,其中(zhong)帶有深色"湔墨(mo)點"的(de)惹(re)人喜愛。三是柳葉(xie)青,色綠(lv)而(er)又(you)帶有朱砂點,石(shi)質堅硬。四是淡綠(lv)色洮(tao)(tao)石(shi),具有滲水緩慢的(de)特點。