洮石(shi)(shi)(shi)(shi)(shi)又(you)稱(cheng)"洮河(he)(he)(he)石(shi)(shi)(shi)(shi)(shi)",以(yi)產于甘(gan)肅(su)省古洮州的(de)洮河(he)(he)(he)沿(yan)岸而(er)得名。洮石(shi)(shi)(shi)(shi)(shi)呈(cheng)綠藍色(se),近似(si)綠松石(shi)(shi)(shi)(shi)(shi)的(de)顏(yan)色(se),多帶回形、曲形紋,質地細膩但不堅,當時人稱(cheng)其為洮硯(yan)。它制成(cheng)的(de)硯(yan)臺稱(cheng)"洮石(shi)(shi)(shi)(shi)(shi)硯(yan)"或"洮硯(yan)",或"洮河(he)(he)(he)石(shi)(shi)(shi)(shi)(shi)硯(yan)"等。質地細膩、致密,晶瑩而(er)有光澤,紋路猶如翻騰的(de)海(hai)浪。
洮石的(de)具體產地在甘肅省甘南藏族自治州卓尼縣洮硯鄉約8公(gong)里的(de)洮河東岸。
化(hua)(hua)學分(fen)析表明,洮(tao)石(shi)含(han)二氧(yang)(yang)化(hua)(hua)硅60.40%,二氧(yang)(yang)化(hua)(hua)鉈0.72%,三(san)氧(yang)(yang)化(hua)(hua)二鋁15.54%,三(san)氧(yang)(yang)化(hua)(hua)二鐵7.74%,氧(yang)(yang)化(hua)(hua)鈣1.19%,氧(yang)(yang)化(hua)(hua)鎂4.56%,氧(yang)(yang)化(hua)(hua)鉀(jia)4.30%,氧(yang)(yang)化(hua)(hua)鈉0.98%.比(bi)重約2.7-2.9,硬度約3-4.洮(tao)石(shi)屬泥(ni)(ni)盆系中(zhong)變質細(xi)泥(ni)(ni)板頁巖,結構細(xi)密,且含(han)多種金屬離子(zi),石(shi)質瑩潤如(ru)玉,叩之無聲(sheng),呵(he)之出水珠,用以制(zhi)硯,貯水不(bu)耗,歷寒不(bu)冰,澀不(bu)留筆(bi),滑(hua)不(bu)拒(ju)墨,且硬度適中(zhong),具有發墨快,研墨細(xi),不(bu)損筆(bi),揮撒自(zi)如(ru),濃淡相宜,用之得心(xin)應手,風雅(ya)盡賦(fu)等(deng)特點(dian)。
洮(tao)(tao)石(shi)(shi)(shi)(shi)(shi)除了綠(lv)(lv)洮(tao)(tao)石(shi)(shi)(shi)(shi)(shi)之外,還有一種(zhong)紅洮(tao)(tao)石(shi)(shi)(shi)(shi)(shi),色彩有些濃艷(yan),比較少,不如綠(lv)(lv)洮(tao)(tao)石(shi)(shi)(shi)(shi)(shi)常見。但綠(lv)(lv)色石(shi)(shi)(shi)(shi)(shi)硯(yan)并非洮(tao)(tao)硯(yan)一家(jia),綠(lv)(lv)端硯(yan)、綠(lv)(lv)松花(hua)石(shi)(shi)(shi)(shi)(shi)硯(yan)、綠(lv)(lv)賀(he)蘭(lan)石(shi)(shi)(shi)(shi)(shi)硯(yan)、綠(lv)(lv)易縣(xian)石(shi)(shi)(shi)(shi)(shi)硯(yan)等各有千秋,區別時要細審(shen)紋(wen)理(li),并看是否有綠(lv)(lv)洮(tao)(tao)石(shi)(shi)(shi)(shi)(shi)中常見的微量黑色墨(mo)筋或墨(mo)點。
制作洮硯的(de)洮石(shi)有(you)(you)(you)數種,一是(shi)鴨頭綠(lv)(lv),也稱"綠(lv)(lv)漪石(shi)",色(se)澤(ze)綠(lv)(lv),有(you)(you)(you)水(shui)波(bo)狀紋(wen)路,石(shi)質(zhi)堅細,瑩潤(run)如玉(yu),是(shi)洮石(shi)上品。如在綠(lv)(lv)色(se)紋(wen)路中(zhong)(zhong)夾雜黃色(se)痕(hen)跡者,則更名貴(gui)。二是(shi)鸚(ying)鵡(wu)綠(lv)(lv),色(se)澤(ze)深(shen)綠(lv)(lv),石(shi)質(zhi)細潤(run),其中(zhong)(zhong)帶(dai)有(you)(you)(you)深(shen)色(se)"湔墨點"的(de)惹人喜愛。三是(shi)柳葉青,色(se)綠(lv)(lv)而又帶(dai)有(you)(you)(you)朱(zhu)砂點,石(shi)質(zhi)堅硬(ying)。四是(shi)淡綠(lv)(lv)色(se)洮石(shi),具有(you)(you)(you)滲水(shui)緩慢的(de)特點。