Just to clarify (it can be confusing), spinel is a member (mineral species) of the spinel group. These are cubic oxides of general formula AE2O4.
The spinel group can be sub-divided into three series:
Spinel series (E = Al), which includes "Spinel" MgAl2O4 (other Al varieties include hercynite, gahnite and galaxite, some of which can also be attractive)
Magnetite series (E = Fe), which includes Magnetite FeFe2O4 (ie Fe3O4)
Chromite series (E = Cr), which includes Chromite FeCr2O4
Magnetite and chromite are the main constituents of "black sand", which is therefore a mixture of spinel group minerals - magnetite is common in lava flows like basalt and chromite is common in serpentinite.
Spinel is found as a metamorphic mineral, but also as a primary mineral in mafic igneous rocks. In these mafic (dark coloured) igneous rocks (basalts. serpentinite), the magmas are relatively deficient in alkalis (Na, Ca, K) relative to aluminium Al. In contrast, light-coloured igneous rocks are high in alkalis and include granite - in which we don't get spinel or corundum.
In the igneous rocks low in alkalies relative to Al, aluminium oxide may form as the mineral corundum [which includes the gems sapphire and ruby] or may combine with magnesium (Mg) to form spinel, which when red is sometimes called "ruby spinel" or "balas ruby" (it is not truly ruby and far less valuable). This is why spinel and ruby and sapphire are often found together, often with lots of black sand.
So the source of spinel, rubies and sapphires are often things like mafic lava flows.
A bit of geological knowledge helps prospecting. For example rocks high in alkalis like granite are high in zirconium Zr, phosphorous P, beryllium Be, boron B and fluorine F (basalts and serpentinite are not), so granite areas often give gem minerals that contain these elements. such as topaz (F), zircon (Zr), emerald and aquamarine (Be) and tourmaline (B). If you are finding things like topaz and sapphire and ruby together, it is probably because the streams are draining basalt flows sitting on top of granite.
Just be careful though,the tin mineral cassiterite (SnO2) can also give you black sand and is associated with granite not basalt or serpentinite - mixtures of topaz, zircon and cassiterite are common in granite areas with no basalt or serpentinite.
You can extend this further, as a coloured gemstone is commonly a common mineral like spinel or corundum that has the bonus of extra traces of minor elements, not part of their chemical formula, such as Cr, Mn, Ni, Ti that give it colour by "doping" it (think of it more like a dye). A type of coarse-grained granite known as a complex pegmatite contains lots of Be (eg Broken Hill district) but for it to produce the gem emerald (rather than just the common mineral beryl that emerald is a green variety of) we need Cr to make it green. So we find emeralds where these pegmatitic granites have intruded serpentinites (since serpentinites are high in Ni and Cr as well as Mg). So we get emeralds near the contact between these rocks at Yalgoo in WA (the Poona emerald), where we would see weathered and pale emeralds scattered around at surface when we were exploring for nickel in the 1960s. The famous emerald fields of South Africa and Zimbabwe (eg Sandawana) occur at the contact between these two rock types. Emeralds from Columbia form a different way (by crystallizing directly into veins from salt water high in Cr and Be associated with salt deposits), and are the most prized for their lighter, finer colour.
The advantage of knowing such things is that you can find undiscovered gemfields - most known gemfields in eastern Australia have been found by gold prospectors, but since granite and basalt areas often contain no gold at all, new fields remain to be found. So it is worth panning and seiving a bit in areas of these rocks that are not in goldfields.
Hope this explains some of the observations that you guys have made here
Cheers hehe
