The chemistry, properties and tests of precious stones

In Short

Written by polymath John Mastin in 1911, this handbook offers a complete guide to the physical and chemical properties of precious stones. Mastin systematically demystifies gemology by detailing how subterranean heat, pressure, and time transform humble elements into diamonds, rubies, and sapphires. He walks readers through the physics of light refraction, crystal structures, hardness scales, and specific gravity tests, while outlining techniques for cutting gems and detecting clever imitations. The volume endures as an exceptionally clear, highly practical bridge between rigorous earth science and the everyday appreciation of fine jewelry.

The Story

The volume opens by resolving a core paradox of gemology: despite their immense commercial value and legendary beauty, precious stones rarely contain rare metals like gold or platinum. Instead, they consist almost entirely of common elements found in the earth's upper crust. Mastin explains that gems owe their existence to slow, natural processes taking place over millions of years. Deep within the earth, water carries dissolved mineral constituents through rock strata, where unceasing pressure and extreme temperatures cause these ordinary elements to crystallize into extraordinary gems.

From this foundational premise, the treatise progresses through a comprehensive survey of the physical laws governing gem classification. Mastin details how minerals naturally organize into defined crystalline systems, building their geometric forms around fixed axes. He demonstrates how these structural arrangements dictate a gem's cleavage planes—the exact lines along which a stone splits when struck—and explains how internal geometry influences optical behavior. Light passing through a crystal undergoes single or double refraction, polarization, or dichroism, producing the vivid play of color seen in fine opals or the fiery brilliance of cut diamonds.

Having established the underlying physics, Mastin shifts focus to practical testing and identification methods used to evaluate gemstones. He explains the Mohs scale of hardness, demonstrating how experts use standard reference minerals or specially tempered steel needles to verify a stone’s identity without causing unnecessary damage. He introduces liquid immersion techniques that group gems by specific gravity, and examines how stones react to heat, magnetism, and electrical charges—noting that amber generates negative electricity when rubbed, whereas cut gems generally develop positive charges.

The work culminates in a detailed examination of gem cutting, synthetic manufacturing, and individual stone profiles. Mastin outlines how lapidaries strategically cut raw minerals to maximize light reflection, eliminate internal blemishes, and preserve maximum weight, highlighting famous historical diamonds like the Cullinan and the Koh-i-Nur as masterworks of the craft. He demystifies the creation of glass "paste" or "strass" imitations and microscopic synthetic diamonds, explaining precisely why man-made processes cannot yet replicate the ages of continuous pressure required to form massive, flawless natural jewels. The text concludes with an exhaustive, stone-by-stone breakdown covering diamonds, sapphires, topazes, zircons, and turquoises, providing readers with exact chemical formulas, color spectrums, and laboratory tests for every major mineral class.

How It Unfolds

The terrestrial origin Mastin establishes that precious gems form from common crustal elements through millions of years of subterranean heat, pressure, and water percolation.

The crystalline foundation The text introduces structural geometry, showing how minerals naturally build themselves around fixed axes into distinct crystal systems that govern their physical behavior.

The dynamics of light and cleavage Detailed chapters explain how structural planes dictate a stone's cleavage and how light behaves as it passes through various gems, creating single or double refraction, dichroism, and opalescence.

The mechanics of lapidary art Mastin explores the physical process of cutting and faceting stones, illustrating how expert gem cutters remove internal flaws and position facets to maximize brilliance.

The detection of counterfeits The author outlines scientific testing methods—including hardness needles, heavy liquid immersion, and heat conduction—to help readers easily distinguish natural gems from glass imitations.

The mineral inventory The final chapters present individual profiles for major gemstones, detailing the unique chemical composition, color variations, and diagnostic test responses for each mineral type.

The People

The Author-Expert John Mastin approaches his subject as an authoritative yet accessible teacher. He seeks to equip both professional diamond merchants and window-shopping amateurs with the precise scientific tools needed to evaluate minerals without being deceived by clever imitations or commercial myths.

The Lapidary Craftsman The gem cutter represents the ultimate practical strategist in the text. Tasked with evaluating raw mineral specimens, the lapidary must carefully calculate cleavage planes and optical angles to carve away blemishes, optimize light reflection, and preserve maximum carat weight.

The Counterfeiter and the Novice The maker of glass "paste" or synthetic stones acts as an ever-present foil to the trained mineralogist. Working with lead-heavy glass formulations or microscopic lab-grown crystals, the imitator attempts to deceive the casual observer, highlighting the crucial need for the novice collector to master scientific testing methods.

In Its Own Voice

"The natural stones represent the slow chemical action of water, decay, and association with, or near, other chemical substances or elements, combined with the action of millions of years of time..."

This foundational observation lays out the core premise of the book, establishing that even the rarest gems are products of common chemical elements transformed by vast geological timescales.

"...so that from the scratch which can be made with the finger-nail on mica, to the hardness of the diamond, which diamond alone will scratch readily, the stones may be picked out, classified and tested, with unerring accuracy."

Writing with characteristic clarity, Mastin emphasizes how practical, methodical testing empowers anyone to evaluate mineral hardness and definitively identify genuine gems.

What It's Really About

At its heart, the book argues that true beauty in the natural world is anchored in fixed physical laws rather than mystical rarity. Mastin repeatedly highlights the quiet wonder of chemistry: the fact that glowing rubies, deep blue sapphires, and brilliant diamonds are forged from the very same common elements that make up ordinary dirt and rock. The text asserts that scientific literacy empowers the everyday collector, removing the mystery from gem valuation by replacing hearsay with empirical testing, chemical analysis, and optical physics. Ultimately, the work champions the idea that understanding the structural geometry and geological origins of a stone deepens, rather than diminishes, our aesthetic appreciation of it.

Why Read It Today

The Chemistry, Properties and Tests of Precious Stones remains a captivating read for gem enthusiasts, mineralogists, and readers fascinated by early twentieth-century science. Mastin writes with an engaging, orderly clarity that makes complex physical concepts—such as double refraction, specific gravity groups, and cleavage planes—instantly accessible to non-specialists. The text provides a fascinating historic look at the state of gemology in 1911, capturing a moment when the industry was responding to historical events like the cutting of the massive Cullinan diamond and the advent of early synthetic gemstones.

Modern readers will appreciate how cleanly the book cuts through market hype, offering straightforward explanations of how stones are identified, tested, and faceted. While some of the chemical notation reflects the conventions of its era and the text includes dense technical tables of specific gravities and hardness indices, these elements only reinforce the book's practical authenticity. It stands as a precise, wonderfully grounded field handbook that turns every jeweler's window display into a fascinating laboratory of natural history.