Metamorphic Or Mineral

Schist

Mica Schist

Yellow Aventurine (Yellow Quartzite)

Silicon Dioxide (SiO2) with inclusions

Garnet Schist

Almandine Garnet in Mica Schist (Fe3Al2(SiO4)3)

Epidote

Epidote - {Ca2}{Al2Fe3+}(Si2O7)(SiO4)O(OH)

Orthoclase Feldspar

KAlSi3O8 (Potassium Aluminum Silicate)

Marble

Metamorphic rock composed of Recrystallized Carbonate Minerals (chiefly CaCO3)

Potassium Feldspar (Orthoclase)

Orthoclase (KAlSi3O8)

Greenschist (Greenstone River Pebble)

Greenschist (Chlorite Schist)

Quartz Vein in Pelite (Metamorphic Rock)

Hydrothermal Quartz Veining in Metasedimentary Rock (SiO2)

Tumbled Moonstone

Feldspar (Orthoclase/Albite), KAlSi3O8

Gneiss (likely)

Gneiss

Yellow Aventurine

Quartz (Silicon Dioxide - SiO2) with inclusions of Mica

Mica Schist with Quartz

Mica-Quartz Schist (Muscovite subclass)

Green Aventurine

Aventurine Quartz (SiO2 + Fuchsite inclusions)

Prehnite

Prehnite (Ca2Al(AlSi3O10)(OH)2)

Orthoclase Feldspar with Quartz

Orthoclase (KAlSi3O8)

Staurolite (Fairy Cross)

Staurolite (Fe2+2Al9Si4O23(OH))

Biotite on Matrix

Biotite (K(Mg,Fe)3AlSi3O10(F,OH)2)

Tiger's Eye

Silicon dioxide (SiO2) with fibrous inclusions of crocidolite

Schist

Schist (various mineralogical compositions including mica, chlorite, talc, etc.)

Black Sand (Magnetite Sand)

Magnetite (Fe3O4)

Mica Schist

Mica Schist (predominantly Muscovite or Biotite sheet silicates)

Tiger's Eye

Pseudomorph of Quartz after Crocidolite (SiO2)

Pyrite

Iron Sulfide (FeS2)

Given the appearance, it's likely a fragment of an igneous or metamorphic rock composed of multiple minerals. Without specific diagnostic features, it's difficult to give a common name other than 'crystalline rock fragment'. If pressed to guess based on the granular, somewhat light and dark speckled appearance, it could be a fragment of granite or a related intrusive igneous rock.

As a rock fragment, it does not have a single scientific mineral name; rather, it's a piece of a rock which is a combination of minerals. If it were granite, its main mineral composition would be quartz (SiO2), feldspar (e.g., KAlSi3O8 - orthoclase, (Na,Ca)(Al,Si)4O8 - plagioclase), and mica (e.g., KAl2(AlSi3O10)(OH)2 - muscovite or K(Mg,Fe)3AlSi3O10(OH)2 - biotite), among others.

Rhodonite

Manganese Silicate (Mn,Fe,Mg,Ca)SiO3

Marble

Metamorphic Limestone (CaCO3)

Rhodonite

Rhodonite (Manganese Iron Magnesium Calcium Silicate - (Mn,Fe,Mg,Ca)SiO3)

Quartz with Muscovite Mica

Silicon Dioxide (SiO2) with Potassium Aluminum Silicate Hydroxide Fluoride (KAl2(AlSi3O10)(F,OH)2)

Rhodonite

Rhodonite (MnSiO3)

Limonite (Iron Ore)

FeO(OH)·nH2O (Hydrous ferric oxide)

Epidote (Unakite-related)

Epidote (Ca2(Al,Fe)3(SiO4)3(OH))

Biotite (Mica)

Biotite (K(Mg,Fe)3AlSi3O10(OH,F)2)

Pyrite

Iron Disulfide (FeS2)

Pyrite (Fool's Gold)

Iron Sulfide (FeS2)

Prehnite

Prehnite (Ca2Al(AlSi3O10)(OH)2)

Natural Gemstone Cabochons (Ruby, Moonstone, Emerald, Citrine)

Ruby (Al2O3:Cr), Moonstone (KAlSi3O8), Emerald (Be3Al2(SiO3)6:Cr), Citrine (SiO2)

Greenschist

Greenschist (Actinolite-Chlorite Schist)

Schist

Mica Schist

Biotite Schist

Biotite Schist

Quartz Vein in Host Rock (Breccia/Rhyolite)

Silicon Dioxide (SiO2) with matrix

Garnet Schist

Almandine Garnet Schist