Diamond

Diamond is chemically the simplest gem on this site — pure carbon, nothing else — and physically the most extreme, sitting at 10 on the Mohs hardness scale, the top of the scale by definition and hard enough that only another diamond can scratch one. That combination of simplicity and extremity is most of why it became the default choice for engagement rings and the reference point every other gem’s hardness gets measured against.

Formation and structure. Diamond crystallizes in the cubic system under the extreme pressure and temperature found roughly 150–200 kilometers below the Earth’s surface, then reaches the surface through volcanic eruptions that form kimberlite and lamproite pipes. The same carbon atoms, under different (lower-pressure) conditions, form graphite instead — one of the clearest examples in mineralogy of how pressure and temperature, not chemistry, can produce completely different materials from identical raw ingredients.

Appearance and color range. Most diamonds sold as gems are graded on a scale from colorless to light yellow or brown, with colorless the traditionally most valuable; “fancy color” diamonds in yellow, pink, blue, and other hues occur naturally through specific trace impurities or structural defects and are valued on an entirely separate, often much higher, scale. Clarity — the presence or absence of internal inclusions and surface blemishes — and cut quality both affect a diamond’s brilliance independently of its color grade.

Hardness and everyday care. At a maximum 10 on the Mohs scale, diamond is by definition the most scratch-resistant gem in daily use, making it the practical top choice for rings worn every day for decades. Hardness is not the same as toughness, however — diamond does have a specific cleavage plane along which a sharp, precisely angled blow can chip or split it, so care is still warranted even though scratching is essentially a non-issue.

Where it is found. Major sources include Botswana, Russia, Canada, Australia, South Africa, and the Democratic Republic of the Congo; laboratory-grown diamonds, chemically and physically identical to natural ones, have also become a significant share of the market and are required to be disclosed as such.

Birthstone role. Diamond is the one April birthstone every system tracked on this site agrees on — the historical list, US 1912, UK 2013, and US current all name it, with only the UK 2013 list adding rock crystal (colorless quartz) as a secondary option. In the modern jewelry-trade zodiac list, diamond is also assigned to Aries; the traditional tropical zodiac instead gives Aries bloodstone, tracing back to the historical March entry for the month Aries begins in.