Corundum: What is that?
Corundum is the name for the “family of gems”
that sapphire and ruby belong in. They are a 9 on the Mohs hardness scale, which means they are hard and scratch resistant when put into rings. Especially when they are cut as cabochons. Star rubies and sapphires need to be cut as cabochons in order to bring out the star. The only gem harder is diamond. Ruby is the most valuable gemstone; even more valuable than diamond because of it’s scarcity. Star sapphire and ruby are rare if natural and untreated. There are lots of man made star cabs on the market, so this is where I recommend a gemologist to check out your purchase and as always buy from a trustworthy dealer.
What makes that star? Rutile are tiny needle like crystals that are present in other stones. Sometimes desirable, sometimes not. These needles orient themselves in the shape of the corundum crystal which is hexagonal. Reflections from these needles create something called chatoyancy; or a streak of light. When the stone is cut into a cabochon the reflection from the needles is concentrated along the top of the stone into three white streaks crossing at 120 degree angles forming a six-rayed star.
Colors and sizes
Sapphire comes in all colors. Blue is the most popular, but you can see star sapphires in grey, purple, pink, black, and varying shades in between. Ruby is red. Ruby is very rare in large clean sizes. Sapphire is easier to find and more abundant. Larger sizes are available. There are great looking stones out there for your collection, and one cannot always tell the difference with the naked eye which stone cost more money. Your friends and relatives don’t walk around with a magnifying glass and neither do I. Besides; that would be rude. Treat yourself. Best regards, Robert.
Continue on to page 2 where we will take a look at some beautiful stars!