Metamorphic Mineral

Schist

Mica Schist

Yellow Aventurine (Yellow Quartzite)

Silicon Dioxide (SiO2) with inclusions

Garnet Schist

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

Orthoclase Feldspar

KAlSi3O8 (Potassium Aluminum Silicate)

Epidote

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

Potassium Feldspar (Orthoclase)

Orthoclase (KAlSi3O8)

Marble

Metamorphic rock composed of Recrystallized Carbonate Minerals (chiefly CaCO3)

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

Green Aventurine

Aventurine Quartz (SiO2 + Fuchsite inclusions)

Yellow Aventurine

Quartz (Silicon Dioxide - SiO2) with inclusions of Mica

Mica Schist with Quartz

Mica-Quartz Schist (Muscovite subclass)

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

Black Sand (Magnetite Sand)

Magnetite (Fe3O4)

Tiger's Eye

Pseudomorph of Quartz after Crocidolite (SiO2)

Pyrite

Iron Sulfide (FeS2)

Mica Schist

Mica Schist (predominantly Muscovite or Biotite sheet silicates)

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.

Marble

Metamorphic Limestone (CaCO3)

Rhodonite

Manganese 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)

Rhodonite

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

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)

Schist

Mica Schist

Greenschist

Greenschist (Actinolite-Chlorite Schist)

Biotite Schist

Biotite Schist

Greenschist

Greenschist

Greenschist (Greenstone)

Greenschist (Metamorphic rock containing chlorite, actinolite, and epidote)

Quartz Vein in Host Rock (Breccia/Rhyolite)

Silicon Dioxide (SiO2) with matrix