der Rubin
ruby
A valuable, usually deep-red gemstone that is a variety of the mineral corundum.
Ihre Halskette war mit einem großen, funkelnden Rubin besetzt.
Her necklace was set with a large, sparkling ruby.

ruby
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ruby
A valuable, usually deep-red gemstone that is a variety of the mineral corundum.
Ihre Halskette war mit einem großen, funkelnden Rubin besetzt.
Her necklace was set with a large, sparkling ruby.

The meaning of 'Rubin' (ruby) can be recognized from its usage. As a countable noun ('ein Rubin' — a ruby, 'zwei Rubine' — two rubies), it refers to the gemstone. As a non-countable color term, often with the preposition 'in', it refers to the color.
Der Dieb stahl die Kette mit den drei Rubinen.
The thief stole the necklace with the three rubies.
As a gemstone, 'Rubin' (ruby) is often used in connection with jewelry and its value. Typical phrases are 'ein Ring mit einem Rubin' (a ring with a ruby), 'mit Rubinen besetzt' (set with rubies), or 'ein wertvoller Rubin' (a valuable ruby).
Er schenkte ihr zum Jubiläum einen Ring mit einem kleinen Rubin.
He gave her a ring with a small ruby for their anniversary.
When 'Rubin' (ruby) describes a color, it often appears with the preposition 'in' or becomes part of an adjective such as 'rubinrot' (ruby-red).
Sie kaufte sich ein Tuch in einem leuchtenden Rubin.
She bought herself a scarf in a bright ruby.
sapphire
'der Rubin' and 'der Saphir' are both valuable gemstones and can also stand for typical colors. They can be confused because both are known as gemstones for jewelry and often appear in rings or necklaces. The most important difference is the color: a ruby is typically red, while a sapphire is typically blue. So if it is about a red stone or a bright red, 'Rubin' fits. For a blue stone or a strong blue, 'Saphir' is more natural.

Im Ring meiner Großmutter glänzt ein kleiner Rubin.
A small ruby shines in my grandmother's ring.
Emma trägt eine Kette mit einem dunkelblauen Saphir.
Emma is wearing a necklace with a dark-blue sapphire.