A lode is a mineral deposit found in the original rock body in which it formed, as opposed to a placer deposit in which gem or metal grains have been weathered, transported and concentrated in alluvium. The word comes from the Old English meaning a course or way, and in mining usage it described a vein or fissure filled with ore that miners followed underground. Modern usage in the gem trade is broader and includes any in-situ source: vein, pegmatite pocket, contact-metamorphic skarn or igneous host rock.

In gem mining context

For coloured stones, the distinction between lode and placer matters because each carries practical implications.

• Lode mining yields stones with their original crystal habit and matrix associations intact. A sapphire crystal from a lode source may carry information about its host rock, its formation environment and its geological age, which lab origin determination uses.
• Placer mining yields stones that have been transported and tumbled. Their corners are rounded, their original crystal faces partly or wholly abraded, and their matrix associations destroyed. They are easier to extract (one washes gravel) but harder to source-identify.

Many of the world's most important coloured-stone districts are mined as both. Sri Lanka's Ratnapura sapphires are predominantly placer; the source lodes have not been definitively identified. Burmese ruby from Mogok includes both placer pits in valley alluvium and lode mining in the Mogok marble metamorphic units. Tanzanian tanzanite at Merelani is exclusively lode-mined, with no economic placer concentration. Brazilian aquamarine and tourmaline are mostly lode-mined from pegmatite pockets, although a small placer trade exists in Minas Gerais river gravels.

Trade implications

For dealers, the lode-versus-placer distinction is part of the provenance picture. Lab origin determination depends partly on the mineral inclusions characteristic of a particular host environment. A stone that has come from a placer source but is sold under a lode-source name is, technically, mislabelled. In practice the trade tolerates considerable looseness here, since most stones do not have intact mining records and the laboratories establish geographic origin from inclusion suites rather than from the dealer's claim.