洮石(shi)(shi)又(you)稱"洮河(he)石(shi)(shi)",以產于甘肅省古(gu)洮州的洮河(he)沿(yan)岸而(er)得(de)名。洮石(shi)(shi)呈綠藍色(se),近似(si)綠松石(shi)(shi)的顏色(se),多帶回形、曲(qu)形紋(wen),質地細膩(ni)但不堅(jian),當(dang)時人稱其(qi)為洮硯(yan)。它制成(cheng)的硯(yan)臺稱"洮石(shi)(shi)硯(yan)"或"洮硯(yan)",或"洮河(he)石(shi)(shi)硯(yan)"等(deng)。質地細膩(ni)、致密,晶瑩(ying)而(er)有光澤,紋(wen)路猶如翻騰的海浪。
洮(tao)石的具體產地在甘肅省甘南藏(zang)族自治州卓尼縣洮(tao)硯(yan)鄉約8公里的洮(tao)河東(dong)岸(an)。
化(hua)(hua)(hua)學分析(xi)表明,洮(tao)石(shi)含二(er)(er)(er)氧(yang)(yang)(yang)(yang)化(hua)(hua)(hua)硅60.40%,二(er)(er)(er)氧(yang)(yang)(yang)(yang)化(hua)(hua)(hua)鉈0.72%,三(san)氧(yang)(yang)(yang)(yang)化(hua)(hua)(hua)二(er)(er)(er)鋁15.54%,三(san)氧(yang)(yang)(yang)(yang)化(hua)(hua)(hua)二(er)(er)(er)鐵(tie)7.74%,氧(yang)(yang)(yang)(yang)化(hua)(hua)(hua)鈣1.19%,氧(yang)(yang)(yang)(yang)化(hua)(hua)(hua)鎂4.56%,氧(yang)(yang)(yang)(yang)化(hua)(hua)(hua)鉀4.30%,氧(yang)(yang)(yang)(yang)化(hua)(hua)(hua)鈉(na)0.98%.比重約(yue)2.7-2.9,硬(ying)度約(yue)3-4.洮(tao)石(shi)屬泥(ni)盆(pen)系中(zhong)變(bian)質(zhi)細(xi)泥(ni)板頁巖,結構細(xi)密,且(qie)含多種(zhong)金(jin)屬離子,石(shi)質(zhi)瑩潤如玉,叩(kou)之(zhi)無聲,呵之(zhi)出水珠(zhu),用以制硯,貯水不耗,歷寒(han)不冰(bing),澀(se)不留(liu)筆,滑不拒墨,且(qie)硬(ying)度適中(zhong),具有(you)發墨快,研墨細(xi),不損筆,揮撒自如,濃(nong)淡(dan)相(xiang)宜,用之(zhi)得心應手,風(feng)雅盡賦等特(te)點(dian)。
洮(tao)石(shi)(shi)除(chu)了綠(lv)洮(tao)石(shi)(shi)之外(wai),還有(you)一(yi)(yi)種紅(hong)洮(tao)石(shi)(shi),色(se)彩有(you)些(xie)濃艷,比(bi)較(jiao)少,不(bu)如(ru)綠(lv)洮(tao)石(shi)(shi)常(chang)見。但綠(lv)色(se)石(shi)(shi)硯并(bing)非洮(tao)硯一(yi)(yi)家,綠(lv)端硯、綠(lv)松(song)花石(shi)(shi)硯、綠(lv)賀蘭石(shi)(shi)硯、綠(lv)易縣石(shi)(shi)硯等(deng)各(ge)有(you)千秋,區別時要細審紋理(li),并(bing)看(kan)是否(fou)有(you)綠(lv)洮(tao)石(shi)(shi)中常(chang)見的微量黑色(se)墨(mo)筋或(huo)墨(mo)點。
制(zhi)作洮硯的洮石(shi)有(you)數種,一是(shi)鴨頭綠(lv)(lv)(lv),也(ye)稱(cheng)"綠(lv)(lv)(lv)漪石(shi)",色(se)澤綠(lv)(lv)(lv),有(you)水波狀紋路,石(shi)質堅細(xi),瑩潤如(ru)玉(yu),是(shi)洮石(shi)上品。如(ru)在綠(lv)(lv)(lv)色(se)紋路中夾雜(za)黃色(se)痕跡者(zhe),則更名貴(gui)。二是(shi)鸚鵡綠(lv)(lv)(lv),色(se)澤深綠(lv)(lv)(lv),石(shi)質細(xi)潤,其(qi)中帶有(you)深色(se)"湔墨點(dian)"的惹(re)人喜愛。三是(shi)柳葉青,色(se)綠(lv)(lv)(lv)而又帶有(you)朱砂點(dian),石(shi)質堅硬(ying)。四(si)是(shi)淡綠(lv)(lv)(lv)色(se)洮石(shi),具有(you)滲(shen)水緩慢的特點(dian)。