Marble is a fine-grained to very coarse-grained metamorphic rock. Marble is often pure white, and if you can see crystals it may look sugary. Marble can be streaked (or marbled) with grey, green, brown or red. It is quite soft and will not scratch glass.
Don’t confuse marble with quartzite which is white and fine grained, but very hard and easily scratches glass.
How was it formed?
Marble is formed from the metamorphism of limestone. Like limestone, marble is mostly made up of the mineral calcite and when powdered it will fizz with white vinegar or dilute acid.