洮(tao)石(shi)又稱"洮(tao)河(he)石(shi)",以產(chan)于甘肅(su)省古洮(tao)州的洮(tao)河(he)沿岸而得名(ming)。洮(tao)石(shi)呈(cheng)綠(lv)藍色,近(jin)似綠(lv)松石(shi)的顏色,多(duo)帶(dai)回形(xing)、曲形(xing)紋,質地細膩但不(bu)堅,當時人稱其為洮(tao)硯(yan)。它(ta)制成的硯(yan)臺稱"洮(tao)石(shi)硯(yan)"或(huo)"洮(tao)硯(yan)",或(huo)"洮(tao)河(he)石(shi)硯(yan)"等。質地細膩、致(zhi)密,晶瑩而有(you)光澤,紋路猶如(ru)翻(fan)騰的海浪。
洮(tao)石的具體產地在甘肅(su)省甘南藏族自治(zhi)州卓尼縣洮(tao)硯鄉約8公里的洮(tao)河東岸。
化(hua)學分析表明(ming),洮石(shi)(shi)(shi)含(han)二(er)(er)氧(yang)(yang)化(hua)硅60.40%,二(er)(er)氧(yang)(yang)化(hua)鉈0.72%,三氧(yang)(yang)化(hua)二(er)(er)鋁15.54%,三氧(yang)(yang)化(hua)二(er)(er)鐵7.74%,氧(yang)(yang)化(hua)鈣1.19%,氧(yang)(yang)化(hua)鎂4.56%,氧(yang)(yang)化(hua)鉀4.30%,氧(yang)(yang)化(hua)鈉0.98%.比重(zhong)約(yue)2.7-2.9,硬度(du)約(yue)3-4.洮石(shi)(shi)(shi)屬(shu)泥盆系中變質(zhi)細泥板(ban)頁(ye)巖,結構細密,且含(han)多(duo)種金屬(shu)離子,石(shi)(shi)(shi)質(zhi)瑩潤如玉,叩之無(wu)聲,呵(he)之出水(shui)珠,用以制(zhi)硯,貯水(shui)不(bu)(bu)(bu)(bu)耗,歷寒不(bu)(bu)(bu)(bu)冰,澀不(bu)(bu)(bu)(bu)留筆(bi),滑不(bu)(bu)(bu)(bu)拒墨(mo),且硬度(du)適中,具有發墨(mo)快,研(yan)墨(mo)細,不(bu)(bu)(bu)(bu)損筆(bi),揮撒自如,濃淡相宜,用之得心應手,風雅盡(jin)賦等特(te)點。
洮(tao)石(shi)(shi)(shi)除(chu)了綠(lv)洮(tao)石(shi)(shi)(shi)之外,還有(you)(you)一種紅洮(tao)石(shi)(shi)(shi),色(se)彩(cai)有(you)(you)些(xie)濃艷,比較少,不如綠(lv)洮(tao)石(shi)(shi)(shi)常見。但綠(lv)色(se)石(shi)(shi)(shi)硯(yan)并(bing)非洮(tao)硯(yan)一家(jia),綠(lv)端硯(yan)、綠(lv)松花石(shi)(shi)(shi)硯(yan)、綠(lv)賀蘭(lan)石(shi)(shi)(shi)硯(yan)、綠(lv)易(yi)縣石(shi)(shi)(shi)硯(yan)等(deng)各有(you)(you)千秋,區別(bie)時要細審紋理,并(bing)看是否(fou)有(you)(you)綠(lv)洮(tao)石(shi)(shi)(shi)中(zhong)常見的微量黑色(se)墨筋或(huo)墨點。
制(zhi)作(zuo)洮(tao)硯的(de)洮(tao)石(shi)有(you)數種,一是(shi)(shi)鴨頭(tou)綠(lv),也稱"綠(lv)漪石(shi)",色(se)(se)(se)澤綠(lv),有(you)水(shui)波狀紋路,石(shi)質(zhi)堅細,瑩(ying)潤如玉,是(shi)(shi)洮(tao)石(shi)上品(pin)。如在綠(lv)色(se)(se)(se)紋路中(zhong)夾雜黃色(se)(se)(se)痕跡者,則更名貴。二是(shi)(shi)鸚鵡(wu)綠(lv),色(se)(se)(se)澤深綠(lv),石(shi)質(zhi)細潤,其(qi)中(zhong)帶有(you)深色(se)(se)(se)"湔(jian)墨點(dian)"的(de)惹人喜愛。三是(shi)(shi)柳葉青,色(se)(se)(se)綠(lv)而又帶有(you)朱(zhu)砂(sha)點(dian),石(shi)質(zhi)堅硬。四(si)是(shi)(shi)淡綠(lv)色(se)(se)(se)洮(tao)石(shi),具有(you)滲水(shui)緩(huan)慢的(de)特點(dian)。