gemstones and other geological materials

see also:

• How Australia evolved in geological terms and evolution
• Victoria's complex and fascinating origins and geology
• gemstone fields of Victoria

• in Victoria, if you have a Miner's Right $28pp and lasts 10 years, you can fossick in the areas outlined in https://resources.vic.gov.au/licensing-approvals/fossicking/where-you-can-prospect-and-fossick note that there are a LOT of rivers and streams where you are NOT allowed to fossick according to https://resources.vic.gov.au/licensing-approvals/fossicking/where-you-can-prospect-and-fossick/rivers-and-streams-where-you-cant-fossick
• unfortunately it seems you are not permitted to fossick in some streams known to have gemstones in the past such as Lerderderg River west of Blackwood, King River near Edi Cutting, Cardinia Creek, etc.

• most gemstones have a hardness value greater than 6 so that it will have sufficient durability or jewelry - and thus if a knife (hardness 5.5-6) can scratch it rather than just leave a surface mark, it has a hardness value less than 6 and is unlikely to be a gemstone

• if you happen to find a crystal with a triangular face or a transparent cube, and it is isometric then you should consider whether it could be a diamond!
  • rough diamond crystals are isometric take several forms including octahedron (double ended pyramid), dodecahedron, triangle, or can be free form, round or cube.
  • triangular sides do appear with other crystal forms as well such as:
    • other isometric crystals eg. garnet, spinel, lapis lazuli, and some metallic crystals are cubes such as galena and pyrite
    • tetragonal eg. zircon, rutile
    • hexagonal eg. quartz (incl. amethyst, citrine), tourmaline, beryl (including acquamarine, emerald), chalcedony (inc. agate, jasper, onyx, prase, petrified wood), corundum (including ruby and sapphire),
• other crystal shapes:
  • orthorhombic eg. topaz, chrysoberyl, olivine (including peridot)
  • monoclinic eg. jade (jadeite and nephrite)
  • triclinic eg. turquoise
  • amorphous eg. opal

• in WA, SA, NT and Tas, by State law meteorites belong to the State and must be donated to the state museum. No such laws are in the other states however, the meteorite is generally the property of the landowner. If the land is government owned, then the meteorite is the property of the government and will go to the local state museum.
• identification
  • magnet test
    • rare earth magnet on a string is the best then bring the rock towards the suspended stationary magnet
    • meteorites nearly always have iron content and should attract a magnet if it doesn't, it is probably not a meteorite but if it does, it could be a meteorite or just an iron ore
  • surface test
    • meteorites usually have no sharp edges, smooth surface which may be slightly rough (not as smooth as river stones), thumb-printing, pin heads of iron on the surface , fusion crusts from atmospheric entry, and if fresh, contraction cracks
    • NB. most meteorites are black initially then will rust into a reddish brown
    • if there are holes on the surface it is not a meteorite but probably of lava origin
  • window test
    • grind a thumbnail size window into the surface of the rock using 40grit or better grinder and make it wet to see the features better
    • look for iron flecks or rounded shapes of chondrules

• these are fossilized imprints of animals (see https://en.wikipedia.org/wiki/Graptolite) which can be found between layers of Ordovician-Carboniferous period slate
• eg. Werribee Gorge

• buried wood which has been converted to rock
• most commonly occurs when trees are buried in fine grained sediments of deltas or flood plains, or volcanic lahars or ash beds
• the woody stems become saturated with mineralised water (usually silica) and if there is insufficient oxygen to cause decay, the minerals replace the cell walls and the void spaces whilst maintaining the architecture of the stem
• depending upon the minerals it may also be agatized, opalized or be petrified by calcite
• carbonized wood is resistant to silicification and is usually petrified by other minerals - especially heavy metals including uranium, selenium, and germanium.
• thus, the presence of concentric circles in a rock consistent with the tree rings makes the identification evident
• the oldest ones are those when vascular trees first evolved on dry land in the Devonian Period some 390 million years ago
• in Victoria, you can visit the remains of a “petrified forest” at Blowholes Road, Cape Bridgewater west of Portland, but it is sandstone which formed layers around the trees without the trees becoming petrified - so there is no petrified wood there and there is nothing worth collecting.

• quartz is silicon dioxide and is insoluble in water at usual temperatures and is relatively inert in crystalline or amorphous forms
• quartz makes up 10% of earth's crust
• amorphous silicon dioxide's melting point is 1,713°C
• silicon dioxide however will dissolve in water when water is a super-critical fluid at above the critical temperature of water 647.096 K (373.946 °C; 705.103 °F) and a pressure of 22.064 megapascals (3,200.1 psi) and when this cools slowly such as at the top of a chamber, the silicon dioxide will crystallize out as quartz - artificial pure quartz crystals up to 1kg in size can be grown this way taking some 1-2 months.
  • at temperatures below 500°C and pressures below 2-3GPa, low quartz tends to form
  • at temperatures above 500°C and pressures 3-4GPa, high quartz tends to form
  • at temperatures above 500°C and pressures below 0.5GPa, tridymite tends to form and at higher temperatures below melting point, cristobalite tends to form
  • at pressures above 2-3GPa and at temperatures above 700°C such as 70km or more deep in earth, the metamorphic rock, coesite inclusions tends to form from quartz but tends to partially revert back to quartz. Coesite also may form by hypervelocity meteorite impact into quartz-bearing rock
  • at pressures above 7.5GPa , such as 660km or more deep in earth in the lower mantle, stishovite with a rutile-like structure tends to form and maybe the main type of silica in the lower mantle but is rare on the earth's surface although can also be formed by hypervelocity meteorite impact into quartz-bearing rock
  • amorphous forms include opal
• tectonic plate movements push supercritical water containing silicon dioxide (and often gold in colloidal form or dissolved as gold salts, and other minerals) up through fault lines towards the earth's surface where it slowly cools to form quartz crystals and quartz reefs
• quartz crystals are piezoelectric and during repeated earthquakes, each creating electric voltages when the quartz is squeezed and this allows gold to precipitate out onto its surface which forms the gold-rich quartz reefs
• as quartz and quartz-containing rocks weather, sand (which is mainly silicon dioxide) is formed which can travel down rivers then ocean currents to form beaches or be blown by winds to create deserts