洮(tao)(tao)石(shi)又(you)稱"洮(tao)(tao)河石(shi)",以產于甘肅省古洮(tao)(tao)州的洮(tao)(tao)河沿岸而得名。洮(tao)(tao)石(shi)呈綠(lv)藍色,近似(si)綠(lv)松石(shi)的顏色,多帶回形、曲(qu)形紋(wen),質(zhi)地(di)細膩但不堅,當時人稱其為洮(tao)(tao)硯(yan)。它制成的硯(yan)臺稱"洮(tao)(tao)石(shi)硯(yan)"或"洮(tao)(tao)硯(yan)",或"洮(tao)(tao)河石(shi)硯(yan)"等(deng)。質(zhi)地(di)細膩、致(zhi)密,晶瑩而有光澤,紋(wen)路猶(you)如翻騰的海浪。
洮(tao)石(shi)的具體產地在甘肅省(sheng)甘南藏族自治(zhi)州卓尼縣洮(tao)硯(yan)鄉約8公里(li)的洮(tao)河(he)東岸。
化(hua)(hua)學分析表明(ming),洮石含二(er)氧(yang)化(hua)(hua)硅60.40%,二(er)氧(yang)化(hua)(hua)鉈0.72%,三氧(yang)化(hua)(hua)二(er)鋁(lv)15.54%,三氧(yang)化(hua)(hua)二(er)鐵7.74%,氧(yang)化(hua)(hua)鈣1.19%,氧(yang)化(hua)(hua)鎂4.56%,氧(yang)化(hua)(hua)鉀(jia)4.30%,氧(yang)化(hua)(hua)鈉(na)0.98%.比(bi)重(zhong)約(yue)2.7-2.9,硬度約(yue)3-4.洮石屬泥盆(pen)系中(zhong)變質細(xi)泥板頁巖,結構(gou)細(xi)密(mi),且含多種金屬離子,石質瑩(ying)潤如(ru)玉,叩(kou)之無(wu)聲(sheng),呵之出水珠(zhu),用以制硯,貯水不耗,歷寒不冰,澀不留(liu)筆,滑不拒墨(mo),且硬度適中(zhong),具有發墨(mo)快,研墨(mo)細(xi),不損筆,揮(hui)撒自如(ru),濃(nong)淡相(xiang)宜(yi),用之得(de)心應手,風雅盡賦(fu)等特點。
洮(tao)(tao)石(shi)(shi)除(chu)了(le)綠(lv)(lv)洮(tao)(tao)石(shi)(shi)之(zhi)外,還有(you)(you)一(yi)(yi)種紅(hong)洮(tao)(tao)石(shi)(shi),色彩有(you)(you)些濃艷,比較少(shao),不如綠(lv)(lv)洮(tao)(tao)石(shi)(shi)常見。但綠(lv)(lv)色石(shi)(shi)硯并非洮(tao)(tao)硯一(yi)(yi)家(jia),綠(lv)(lv)端硯、綠(lv)(lv)松花石(shi)(shi)硯、綠(lv)(lv)賀蘭石(shi)(shi)硯、綠(lv)(lv)易縣(xian)石(shi)(shi)硯等各有(you)(you)千秋,區別(bie)時要(yao)細(xi)審紋理(li),并看是否有(you)(you)綠(lv)(lv)洮(tao)(tao)石(shi)(shi)中常見的微(wei)量(liang)黑色墨筋或墨點。
制作洮硯的(de)(de)(de)洮石(shi)(shi)(shi)(shi)(shi)有(you)數種,一是(shi)鴨頭綠(lv)(lv)(lv),也(ye)稱"綠(lv)(lv)(lv)漪石(shi)(shi)(shi)(shi)(shi)",色(se)(se)澤綠(lv)(lv)(lv),有(you)水(shui)波狀(zhuang)紋路,石(shi)(shi)(shi)(shi)(shi)質堅細,瑩潤如玉(yu),是(shi)洮石(shi)(shi)(shi)(shi)(shi)上品。如在綠(lv)(lv)(lv)色(se)(se)紋路中夾(jia)雜黃(huang)色(se)(se)痕(hen)跡者,則更名貴。二(er)是(shi)鸚鵡綠(lv)(lv)(lv),色(se)(se)澤深綠(lv)(lv)(lv),石(shi)(shi)(shi)(shi)(shi)質細潤,其中帶(dai)有(you)深色(se)(se)"湔墨點"的(de)(de)(de)惹人喜愛(ai)。三是(shi)柳葉青,色(se)(se)綠(lv)(lv)(lv)而(er)又(you)帶(dai)有(you)朱砂點,石(shi)(shi)(shi)(shi)(shi)質堅硬。四是(shi)淡綠(lv)(lv)(lv)色(se)(se)洮石(shi)(shi)(shi)(shi)(shi),具(ju)有(you)滲水(shui)緩慢的(de)(de)(de)特(te)點。