洮(tao)(tao)(tao)(tao)石(shi)(shi)(shi)又稱"洮(tao)(tao)(tao)(tao)河(he)石(shi)(shi)(shi)",以產于甘肅(su)省古洮(tao)(tao)(tao)(tao)州的(de)(de)洮(tao)(tao)(tao)(tao)河(he)沿(yan)岸而得名。洮(tao)(tao)(tao)(tao)石(shi)(shi)(shi)呈綠藍色(se),近似(si)綠松石(shi)(shi)(shi)的(de)(de)顏色(se),多帶回(hui)形、曲(qu)形紋(wen)(wen),質地(di)細膩(ni)但不堅,當(dang)時人稱其為洮(tao)(tao)(tao)(tao)硯(yan)(yan)(yan)。它制成的(de)(de)硯(yan)(yan)(yan)臺稱"洮(tao)(tao)(tao)(tao)石(shi)(shi)(shi)硯(yan)(yan)(yan)"或(huo)"洮(tao)(tao)(tao)(tao)硯(yan)(yan)(yan)",或(huo)"洮(tao)(tao)(tao)(tao)河(he)石(shi)(shi)(shi)硯(yan)(yan)(yan)"等。質地(di)細膩(ni)、致密,晶瑩而有光澤,紋(wen)(wen)路猶如翻騰的(de)(de)海浪。
洮石的(de)具(ju)體產(chan)地在甘肅省甘南藏族自治(zhi)州(zhou)卓尼(ni)縣洮硯鄉約8公里的(de)洮河東岸。
化學分(fen)析表明,洮石含(han)(han)二氧(yang)化硅60.40%,二氧(yang)化鉈0.72%,三氧(yang)化二鋁15.54%,三氧(yang)化二鐵7.74%,氧(yang)化鈣1.19%,氧(yang)化鎂4.56%,氧(yang)化鉀4.30%,氧(yang)化鈉0.98%.比重約(yue)2.7-2.9,硬度(du)約(yue)3-4.洮石屬泥盆系中變質(zhi)細(xi)泥板頁巖(yan),結構細(xi)密(mi),且含(han)(han)多種金屬離子,石質(zhi)瑩潤如玉,叩之(zhi)(zhi)無聲,呵(he)之(zhi)(zhi)出(chu)水珠,用以制硯,貯水不耗,歷寒不冰,澀不留筆,滑不拒墨(mo),且硬度(du)適中,具有(you)發墨(mo)快,研墨(mo)細(xi),不損筆,揮撒自如,濃淡相宜,用之(zhi)(zhi)得心應(ying)手,風雅盡賦等特點。
洮(tao)石(shi)(shi)(shi)除了綠洮(tao)石(shi)(shi)(shi)之外,還(huan)有(you)一種紅洮(tao)石(shi)(shi)(shi),色(se)彩(cai)有(you)些濃艷,比(bi)較少,不如綠洮(tao)石(shi)(shi)(shi)常見。但綠色(se)石(shi)(shi)(shi)硯(yan)并(bing)非(fei)洮(tao)硯(yan)一家,綠端(duan)硯(yan)、綠松花石(shi)(shi)(shi)硯(yan)、綠賀蘭石(shi)(shi)(shi)硯(yan)、綠易(yi)縣(xian)石(shi)(shi)(shi)硯(yan)等各有(you)千秋,區別(bie)時(shi)要細審(shen)紋(wen)理,并(bing)看是否(fou)有(you)綠洮(tao)石(shi)(shi)(shi)中(zhong)常見的微量(liang)黑色(se)墨(mo)筋或(huo)墨(mo)點。
制作洮硯(yan)的(de)洮石(shi)(shi)有數(shu)種,一是(shi)(shi)(shi)鴨頭綠(lv),也(ye)稱(cheng)"綠(lv)漪石(shi)(shi)",色(se)(se)澤綠(lv),有水(shui)波(bo)狀紋路,石(shi)(shi)質(zhi)堅細(xi)(xi),瑩潤如玉,是(shi)(shi)(shi)洮石(shi)(shi)上(shang)品。如在綠(lv)色(se)(se)紋路中夾雜黃色(se)(se)痕(hen)跡者,則更名貴(gui)。二是(shi)(shi)(shi)鸚鵡綠(lv),色(se)(se)澤深綠(lv),石(shi)(shi)質(zhi)細(xi)(xi)潤,其中帶(dai)有深色(se)(se)"湔墨點(dian)"的(de)惹人喜愛。三是(shi)(shi)(shi)柳葉青,色(se)(se)綠(lv)而(er)又(you)帶(dai)有朱砂點(dian),石(shi)(shi)質(zhi)堅硬。四是(shi)(shi)(shi)淡(dan)綠(lv)色(se)(se)洮石(shi)(shi),具有滲水(shui)緩慢的(de)特(te)點(dian)。