Tortola is the main island of the British Virgin Islands. The beaches are good enough, but not the best in the Caribbean. But anyway, the island is beautiful and worth to spend at least a few days. It is better to have a car, since public transportation is very poor. Shore and seabed: sand, stones. 
Choice of hotels is limited in Tortola, while rooms' fares range from US$150 to US$300 per night. However, service is of a very high standards in most cases. In addition, the place is rather exclusive, because there are no crowds of tourists.