Opal merupakan permata yang setara atau bahkan dapat lebih berharga daripada intan. Di Indonesia, opal permata dijumpai di Kabupaten Lebak, Provinsi Banten. Opal Banten dikenal karena keindahan permainan warnanya. Tulisan ini merupakan ulas balik hasil penelitian terdahulu yang berupa penelitian awal pembentukan opal Banten, karakteristik opal-CT, dan penentuan tipe opal berdasarkan analisis geokimia serta ditambah beberapa data baru, sehingga dapat dibuat model mineralisasinya. Penelitian yang telah dilakukan meliputi penelitian lapangan dan analisis mineralogi/gemologi, petrografi, X-RD , serta geokimia unsur utama dan unsur jarang. Opal Banten termasuk opal-CT yang menunjukkan permainan warna bagus. Opal ini terjadi karena proses pelapukan dan pelindian silika, sehingga membentuk opal pada batulempung abu-abu gelap. Model pembentukan terbagi dalam tiga periode; pada Pliosen Awal terjadi pengendapan batuan vulkanik klastika yang kaya gelas vulkanik sebagai endapan fluviatil. Setelah itu pada Pliosen Akhir-Plistosen terjadi proses pelipatan, pelapukan, dan pelindian silika. Retakan, pelipatan, dan patahan yang terjadi mempercepat proses pelapukan dan pelindian, sehingga terbentuk opal pada sayap lipatan hingga Holosen. Daerah prospek opal terdapat pada satuan tuf bersisipan konglomerat dan breksi pumis, di sayap antiklin. Batuan pembawa opal adalah batulempung berwarna abu-abu gelap yang mengalasi konglomerat polimik/batupasir kerakalan dengan struktur silang-siur dan erosional, dengan kedalaman lebih dari 8 m.

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Granitoid rocks which found at Luk Ulo melange complex as rock fragments with  pale gray colour and faneritic texture. Petrogenesis and geotectonic of the granitoid is under debate. Some geologists consider as plagiogranite, which is formed from the Mid Ocean-ridge (MOR); or leucogranite which is formed from continental collision, and others argue as arc-related granitoid type.The field studies ware carried out on 5 (five) tracks around Luk Ulo River and 1 (one) track at Lokidang River. The pale grey Karangsambung granitoid is composed of the mainly  K-feldspar (34-55%), plagioclase (10-25%) and quartz (25-35%), and chemically contains SiO2 (61.25 - 66.06%); Al2O3 (13.94 – 14.61%), K2O (2.53 - 4.00%), Na2O (3.42 - 4.10%), CaO (2.32 - 4.76%), Fe2O3 total (5.85 – 8.71%), MgO (0.98 – 1.97%). The granitoid is M- and I-type that were formed at 760o - 800o C with a depth of about 20-30 km, resulting from the differentiation of magma from a fragment origin of the K-enriched oceanic island arc originating from drifting of the IAB fragment. The sample of basalt 17D has a relatively high of Nb/Ta ratio (20), low Rb (<2 ppm), low Ba (17 ppm), and is interpreted as interacting with MORB mantle magma containing rutile-melt;whereas quartz monzonite (17A) has a relatively low of Zr/Sm ratio (3.86), which is indicated to have been contaminated by a carbonatite magma. The spidergram pattern of mantle metagabbro (sample no. 13) similar with the basalt from IAB-Bransfield Strait (Antarctica). Results of a comprehensive geochemical study proposes that the current condition of the Karangsambung zone is part of geotectonic of ACM-Eurasia, that composed of a combination of four rock fragments, i.e. (a) the rocks which sourced from IAB fragments, (b) mantle MORB, (c) continental crust from the origin of ACM-Eurasia, (d) the origin fragment from carbonatite magma.Keyword: Luk Ulo Melange Complex, pale grey granitoid, Island-arc granitoid, M  and I-type granitoid

Geomorphology is a landform that extends on the surface of the earth as a result of interactions between endogenous and exogenous processes. Landform classification based on morphometry, morphogenesis, and morphoarengement. Kebumen has attractive geological diversity. The potential of iron sand spread out as beach sand deposits. This research conducted to map landforms, including south coast region and analyze the distribution of iron sand sediment. The study used DEMNAS image analysis, a 1: 20, 000 scale with Arc-GIS software. Variables analysis includes elevation, aspect, roughness, hill shade, river pattern, and hill roughness accompanied by a field survey. Resistivity survey on old beach deposits morphology (2 locations) and young beach deposits (2 locations) to obtain data on the potential of iron sand. Measurement using OYO Model 2, McOHM Resitivimeter, Schlumberger configuration, and geophysical modelling using Res-2Din software. The landform in Kebumen consists of 33 units as structural, denudational, dissolving, fluvial, and coastal landforms. The Coastal Sediment landscape consists of 3 units, in the form of young coastal sediment, old coastal sediment, and fluvio marine sediment. The potential of iron sand is founded in young coastal sediment (M5) is about 764, 77 Ha, and old coastal sediment (M4) about 590, 84 Ha. Mineral compositions are olivine, pyroxene, hornblende, biotite, and impurities as quartz, plagioclase, orthoclase, rutile and calcite minerals. Coastal sand sediment in the Old Sediment is found 30-60 m depth with overburden up to 4 m. The potential of beach sand about 406, 686, 300 m3 similar with 1, 037, 050, 065 tons, total Fe estimated about 629, 696, 799 tons.