Gemstones: Prices, Sources and Mines

What makes a mineral deposit valuable

A valuable mineral deposit is one that can be mined at a profit, which is a higher bar than simply containing something valuable. For metals, the usual tests are grade (how much metal per tonne of rock), tonnage, depth and shape, how easily the ore can be processed, and the cost of getting the product to market. Add the price of the commodity, the permits, and the politics of the country, and a deposit that’s worthless today can become a mine when prices rise or a road gets built.

Gem deposits follow different rules. The gems themselves are a tiny part of the rock, and two stones of the same size can differ in value by a factor of a hundred or more because of color and clarity. A gem deposit’s value depends on how many stones of saleable quality it yields per tonne of rock or gravel moved, and how consistently it does so, which is why gem miners talk about bulk sampling and recovery rather than average grade. Deposits also run out: the Argyle mine in Western Australia, long the main source of pink diamonds, closed in 2020 when its economic ore was exhausted.

How gemstone deposits form

Most gemstones form in a small number of geological settings, and knowing which one a gem comes from is the starting point for finding more of it.

Placers matter because hard, dense gems survive erosion and collect in gravels long after the rock they formed in has worn away. Many famous gem fields are alluvial, and the first clue to a hard-rock source is often a trail of gems or indicator minerals in a river. Our pieces on tanzanite mining at Merelani and Sandawana emeralds look at two single-source gems.

What sets a gemstone’s price

Colored gemstones are priced on much the same factors as diamonds: color, clarity, cut and carat weight. For most colored stones, color carries the most weight, and the most valuable hue, saturation and tone are specific to each gem. Large stones are rarer than small ones, so the price per carat climbs steeply with size. Origin can add a premium when it’s documented, as with Colombian emeralds or Kashmir sapphires.

Treatment matters as much as anything. Many rubies and sapphires are heated, and most emeralds are oiled to hide fractures. These are accepted practices but must be disclosed, and an untreated stone of the same appearance sells for more. For an expensive purchase, a report from an independent gem lab confirms the species, any treatment and, where possible, the origin. Price guides in this hub, such as painite price per carat and padparadscha sapphire value, show how wide the range is for a single gem.

Exploring for gemstones

Exploration for gems starts with the host rock, because the gems themselves are too sparse to target directly. Diamond explorers look for kimberlite pipes using magnetic surveys and indicator minerals such as pyrope garnet and chrome diopside in stream and soil samples. Pegmatite and emerald explorers map the right granite and schist contacts. Alluvial projects look for old river channels and terraces where gravels have collected. In many gem regions, small-scale and artisanal diggers find a deposit before any company does, so mapping where they work, and talking to them, is often the quickest early lead.

Before you buy a gem mine or claim

Gem mines and claims change hands often, and the claims made in sales listings are hard to check. Before buying, confirm that the license or claim is valid, in the seller’s name and in good standing, and what the rules are on exporting rough stones. Ask for production records over several years and check them against sales, because a handful of good stones can make a poor deposit look rich. A bulk sample processed under your own supervision is worth more than any report. Our checklist for buying a sapphire or opal mine covers the details. For hobbyists, fee-dig sites such as the public gem mines in Texas are the easier way to start.